Dendrochronology, the study of tree-ring records to glean information about the past, is possible because each year a tree adds a new layer of wood between the existing wood and the bark. In temperate and subpolar climates, cells added at the growing season's start are large and thin-walled, but later the new cells that develop are smaller and thick-walled; the growing season is followed by a period of dormancy. When a tree trunk is viewed in cross section, a boundary line is normally visible between the small-celled wood added at the end of the growing season in the previous year and the large-celled spring wood of the following year's growing season. The annual growth pattern appears as a series of larger and larger rings. In wet years rings are broad; during drought years they are narrow, since the trees grow less. Often, ring patterns of dead trees of different, but overlapping, ages can be correlated to provide an extended index of past climate conditions.

However, trees that grew in areas with a steady supply of groundwater show little variation in ring width from year to year; these "complacent" rings tell nothing about changes in climate. And trees in extremely dry regions may go a year or two without adding any rings, thereby introducing uncertainties into the count. Certain species sometimes add more than one ring in a single year, when growth halts temporarily and then starts again.

1. The passage suggests which of the following about the ring patterns of two trees that grew in the same area and that were of different, but overlapping, ages?
A. The rings corresponding to the overlapping years would often exhibit similar patterns.
B. The rings corresponding to the years in which only one of the trees was alive would not reliably indicate the climate conditions of those years.
C. The rings corresponding to the overlapping years would exhibit similar patterns only if the trees were of the same species.
D. The rings corresponding to the overlapping years could not be complacent rings.
E. The rings corresponding to the overlapping years would provide a more reliable index of dry climate conditions than of wet conditions.

OA:A

the relevant part of the passage says ...

Often, ring patterns of dead trees of different, but overlapping, ages can be correlated to provide an extended index of past climate conditions.

Q1: the answer is A. "overlapping" is mentioned in the last sentence of the first paragraph, which tells us that "Often, ring patterns of dead trees of different, but overlapping, ages can be correlated to provide an extended index of past climate conditions" 

From the beginning of the first paragraph, we learn that studying ring patterns can teach us about climate conditions during those years. This last sentence tells us that several trees with overlapping ages can prvide an extended index - meaning that we can find the climate conditions for a longer time than the life of a single tree. We can infer that the overlapping is important - it allows us to say "these tree grew at the same time", so we can add the "timeline" of climate conditions of both trees together to create our extended index. Since the overlap is the thing that "connects" the two trees, we must assume that the rings created in the overlapping years will be the similiar, allowing the investigator to correlate the two differnt trees to the same time period. 

2. In the highlighted text, "uncertainties" refers to
A. dendrochronologists' failure to consider the prevalence of erratic weather patterns
B. inconsistencies introduced because of changes in methodology
C. some tree species' tendency to deviate from the norm
D. the lack of detectable variation in trees with complacent rings
E. the lack of perfect correlation between the number of a tree's rings and its age

OA:E

The topic of the passage is "the study of tree-ring records to glean information about the past". the first paragraph then goes on to detail how each year a tree adds a new layer of wood between the wood and the bark, with different rings corresponding to different climate: for the beginning of the growing season, the rings are large and thin-walled and later become small and thick walled. The exact process is unimportant - it's important to understand that the passage tells you what you can learn from the predictable growth pattern of tree rings. 

The second paragraph begins with "however", signaling a change in direction from the first. This paragraph then goes on to show cases where studying tree rings cannot teach us any information, because the predictable patterns described in the first paragraph are disturbed: for example, trees in an area with a steady supply of groundwater show little variation in ring width (as opposed to the "large and thin -->small and thick" variation we described in the first paragraph). The next sentence , which includes the "uncertainties", continues the same direction: in extremely dry regions, a tree won't add any rings - so the predictable pattern of ring growth from year to year is disturbed, so the information we glean from the tree ring patterns is less certain. - specifically, a tree may be older than the number of his rings suggest, since he grows for an extra year or two without adding rings.


3. The passage is primarily concerned with
A. evaluating the effect of climate on the growth of trees of different species
B. questioning the validity of a method used to study tree-ring records
C. explaining how climatic conditions can be deduced from tree-ring patterns
D. outlining the relation between tree size and cell structure within the tree
E. tracing the development of a scientific method of analyzing tree-ring patterns

OA:C

"tracing the development of a scientific method..." implies that the passage is giving a history or a description of the origins of the method itself. that's definitely not happening here.

"Contradiction" is far too strong, and essentially inaccurate. The second paragraph acknowledges some limitations of the tree-ring method, but in no way calls into question the validity of that method as a whole.

 If the choice said "explaining and qualifying...", then, sure.

 Still, it's clear that the primary purpose of the passage is to explain how/why the method works. The second paragraph is basically a "disclaimer", not unlike the medical warnings at the end of drug commercials (whose primary purpose is to sell the drugnot to provide warnings!)