Subject Verb Agreement Question

[DanqingY490]

Here is the question.

"The period (when the great painted caves at lascaux and Altamira were occupied by Upper Paleolithic people has been stablished by carbon-14 dating, but what is much more difficult to determine are the reason for their decoration, the use to which primitive people but caves, and the meaning of the magnificently depicted animals.
A. has been established by carbon - 13 dating, but what is much more difficult to determine are
B. has been established by carbon - 13 dating, but what is much more difficult to determine is
C.have been established by carbon - 13 dating, but what is much more difficult to determine is
D. have been established by carbon - 13 dating, but what is much more difficult to determine are
E. are by carbon - 13 dating, but what is much more difficult to determine is"



This is a question appeared in Ron's 2009 Thursday session.
Ron mentioned the subject for the later part of the sentence is "What is much more difficult to determine"
My questions why is What... a subject? If What ... is subject why it is followed by are rather than is?

I got confused because:
The beginning of the lecture he mentioned a subordinate clause cannot be the subject.
So here I assume what ... cannot be subject as a result it is a Backward construction of the later list being the subject, and select determin are.

Even if What... is subject, why is it plural not singular?

[RonPurewal]

the subject is indeed "what xxxx".

that subject is singular, because "what xxxx" clauses are ALWAYS singular.
(the correct sentence is "what is more difficult to determine is xxxx".)

[aflaamM589]

Hello Ron,
Does it also depend on what follows the what clause a?; i.e


what is much more difficult to determine is the reason blah blah--> correct

what is much more difficult to determine are the reasons blah blah--> also correct,yeah ?

Many thanks

[RonPurewal]

no.

[aflaamM589]

no.

what is much more difficult to determine is the reason blah blah--> correct

what is much more difficult to determine are the reasons blah blah--> also

what is much more difficult to determine is the reasons blah blah.
-->now correct, right?

: )

[RonPurewal]

at this point you've gotten well into the territory of things that are completely irrelevant to the gmat exam.

[aflaamM589]

Ok .
Thanks : )

[RonPurewal]

sure.