Navigators have known for thousands of years that the ocean
has variable currents, but it is only in the last half century that
a reasonably clear picture has emerged of the patterns and
causes of ocean currents.

A. a reasonably clear picture has emerged of the patterns
and causes of ocean currents.
B. a reasonably clear picture of the patterns of ocean
currents and their causes have emerged.
C. a reasonably clear picture emerged of ocean currents,
their patterns and the causes of them
D. there have emerged a reasonably clear picture of the
patterns of ocean currents and what caused them.
E. there had emerged a reasonably clear picture of the
patterns of ocean currents and their causes

This is how you can solve this question in less than a minute:

Concept:
--> You have a subordinate clause starting form THAT... ( make sure you know the difference b/w a main clause and a subordinate clause)

--> This question deals with the concept of subject verb agreement. Also, the subject in few of the answer choices are inverted.

What do I mean by inverted subject?
Eg: The king sat on the bench.
Inverted: On the bench sat the king.

Solution:

Each subordinate clause can have it's own subject and verb. In this case that subject is : A reasonably clear picture ( the highlighted part is the adjective, not the subject).

Since the subject is singular we can have either of the two grammatical constructions:
1. A reasonably clear picture HAS emerged.
2. There HAS emerged a reasonably clear picture. ( Inverted subject)

B: Does not follow this construction
C: It's a fragment, the main verb is missing.
D: Does not follow the inverted subject construction.
E: Incorrect tense form + other issues. ( It's quite unlikely that anyone of you would be confused about this answer choice, however if you still want me to explain I'll be happy to provide an explanation-just ask for it)

Inverted constructions are tested often on the GMAT. The moment you realize that this in an inverted S-V question you can get down to the correct answer pretty quickly.

eybrj2, hope this helps
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i pick A. B is wordy, in my point of view C, D and E are nonsensical and a bit awkward. correct me if i am wrong. by the way whats the source of this question?
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Navigators have known for thousands of years that the ocean has variable currents, but it is only in the last half century that a reasonably clear picture has emerged of the patterns and causes of ocean currents.

A. a reasonably clear picture has emerged of the patterns and causes of ocean currents.
B. a reasonably clear picture of the patterns of ocean currents and their causes have emerged.
C. a reasonably clear picture emerged of ocean currents, their patterns and the causes of them
D. there have emerged a reasonably clear picture of the patterns of ocean currents and what caused them.
E. there had emerged a reasonably clear picture of the patterns of ocean currents and their causes

First let us break the SC into clauses

1.Navigators have known for thousands of years
2.the ocean has variable currents,
3.but it is only in the last half century
4.a reasonably clear picture has emerged of the patterns and causes of ocean currents.

In the above clauses the 4 th clause is the underlined portion.

Process of elimination.

Choice B . a reasonably clear picture of the patterns of ocean currents and their causes have emerged -in the 4 th clause the subject is "clear picture" so the verb should be singular but in the option "have emerged" is plural and "theirs" referring to "clear picture".So B is wrong

Choice C a reasonably clear picture emerged of ocean currents, their patterns and the causes of them - here in this we do not have any verb reffering to the subject of the clause "clear picture".So C is incorrect

Choice the subject of the clause "clear picture" is singular but the verb "have is plural" so D is wrong

Choice D "their" which is plural refers to subject of the clause "clear picture" which is singular so E is wrong


The final answer is "A"

I hope the explanation helps.Please correct if I am wrong.
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