Sentence Correction

The period when the great painted caves at Lascaux and Altamira were occupied by Upper Paleolithic people has been established by carbon-14 dating，but what is much more difficult to determine are the reason for their decoration，the use to which primitive people put the caves，and the meaning of the magnificently depicted animals．
(A) has been established by carbon-14 dating，but what is much more difficult to determine are
(B) has been established by carbon-14 dating，but what is much more difficult to determine is
(C) have been established by carbon-14 dating，but what is much more difficult to determine is
(D) have been established by carbon-14 dating，but what is much more difficult to determine are
(E) are established by carbon-14 dating，but that which is much more difficult to determine is


Why option B is wrong?


We need what is much difficult are the A,B&,C. Am I right?

vishalwin wrote:
Why option B is wrong?

We need what is much difficult are the A,B&,C. Am I right?

Actually option B is the OA.

The first tipoff that you need is rather than are is that the clause starts with what is rather than with what are. What is ... are... does not seem logical.

Then there is some obscure rule that says that you use the verb form that matches the first member of the list, which is a compound noun.

I say go with the first reason, as that second reason is too arcane. When in doubt hack your way to correct SC answers.

A clause may serve as a NOUN.
More specifically, a clause may serve as a SUBJECT.
A clause serving as the subject of a verb is known as a SUBJECT CLAUSE.
A subject clause is ALWAYS SINGULAR.

OA: What is much more difficult to determine is the reason for their decoration, the use to which primitive people put the caves, and the meaning of the magnificently depicted animals.
Here, the portion in red is a SUBJECT CLAUSE in that it contains both a subject and a verb (what is) and serves as subject for the verb in blue (the second is).

A and D: What is much more difficult to determine are the reason for their decoration, the use to which primitive people put the caves, and the meaning of the magnificently depicted animals.
Here, are (plural) does not agree with what is much more difficult to determine (a subject clause, which by definition is considered singular).
Eliminate A and D.

@GMATGuruNY Your explanation are amazing. Thanks.