Geologists Harris and Gass hypothesized that the Red Sea rift develope

Before diving into the questions, here's a breakdown of the passage as a whole.

Paragraph 1: the author presents the viewpoint of geologists Harris and Gass (H&G)

• H&G's main hypothesis is that "the Red Sea rift developed along the line of a suture."
  
  • The author then defines suture as "a splice in the Earth’s crust" -- in other words, different kinds of crust material smashed together at this location.
• H&G's evidence to support this claim is that there are differences in the upper layers of rocks on either side of the rift, which hint at differences in the rocks lower down.

Paragraph 2: the author presents an opposing viewpoint held by other geologists:

• These geologists believe that "there is inadequate evidence to conclude that a suture underlies the rift."
• The evidence for this viewpoint is that "neither the upper rock layer nor the underlying igneous rocks on the one side of the rift differ fundamentally from the corresponding layers on the other side."

So, the "other geologists" think that there is no suture, because the rocks on both sides of the rift are basically the same.

Paragraph 3: H&G get the final word. They think that the other geologists are wrong because:

• "the upper rock layers on the two sides of the rift had not been shown to be of similar age, structure, or geochemical content," and because
• "the underlying igneous rocks on either side of the rift contain significantly different kinds of rare metals."
  
  In all, H&G present more evidence that there IS a suture. Their evidence is that the rocks ARE different on each side of the rift.

Now, let's take a look at the questions:

Question 1

We're looking for the answer choice that WEAKENS H&G's hypothesis. Remember that their hypothesis depends on the rocks on one side of the rift being different than the rocks on the other side of the rift.

So, which piece of info about the upper layers of rock would weaken H&G's hypothesis?
H&G don't think that the upper layers of rock caused a suture to develop -- they just think that the composition of the upper layers provides evidence that there is a suture underneath.

Because the hypothesis doesn't depend on the upper layers causing a suture, (A) doesn't weaken the argument.

H&G don't care about where a suture WILL occur, and their hypothesis doesn't rely on the upper layers leading to a suture -- instead, they just think that these particular layers hint at differences in the rocks underneath, which in turn hints at the existence of a suture.

Since the suture already exists, (B) has no impact on H&G's hypothesis. Eliminate (B).

Here we go. H&G think that the composition of the upper rocks "give[s] clues" about the nature of the underlying rocks. (C) weakens this line of thought by saying that, nope, the upper layer of rocks CANNOT provide information about the nature of the underlying rocks.

(C) weakens H&G's hypothesis by taking away one of their pieces of supporting evidence. Keep (C).

(D) actually STRENGTHENS H&G's hypothesis -- they think that the rocks are different, which shows that there is a suture. (D) tells us that if the rocks are different, then the ONLY explanation is that there is a suture.

(D) is out.

H&G base their hypothesis on the upper layers on one side of the rift being different from the upper layers on the other side of the rift. These differences hint at differences in the rocks below.

Imagine, as an example, that a certain baker usually puts a layer of chocolate cake under a layer of vanilla cake. She also usually puts a layer of lemon cake under a layer of strawberry cake.

H&G argue that the upper layers at the rift are vanilla on one side and strawberry on the other, which means that the lower layers are probably chocolate on one side and lemon on the other. Because chocolate and lemon are different, the two sides must have been spliced together.

(E) tells us that upper layers in general are different than lower layers in general. This has nothing to do with H&G's argument -- it doesn't give us further information to determine whether there is a suture beneath the upper layers.

Going back to the baking example, (E) just says that the baker likes to put different layers on top of one another. It gives us no insight about whether the sides of the rift are the same, or are different -- maybe the top layers are vanilla on both sides, or maybe they are vanilla and strawberry.

Because (E) doesn't support or weaken H&G's hypothesis, we can get rid of it.

Question 2

Remember that the "other geologists" believe that there is NOT a suture. Their support for this argument is that the rocks on both sides of the rift are the same.

Nope, the "other geologists" think that there is NOT a suture.

(A) is out.

We don't have enough info to say that the geologists would agree with (B). They think that there's not enough evidence to conclude that there's a suture in THIS case, but the passage doesn't tell us what they believe about finding OTHER sutures. Maybe there is technology that would allow them to discover a suture that's covered up by other geological features.

We don't know that the geologists would agree with (B), so cross it out.

Again, we have no information to support (C). The geologists argue that a certain rift is not a suture -- we have no idea what they think about how rifts are developed in the first place.

Eliminate (C).

... No. Again, the "other geologists" don't weigh in on the issue of dating rocks at all.

(D) is out. Hopefully (E) is ok...

Whew, this one looks good. It aligns nicely with the geologist's argument -- they think that the rocks on both sides of the rift are the same, and that therefore we can't conclude that there is a suture.

(E) is the correct answer to question 2.

I hope that helps!