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


I chose A because after reading the NON-underlined section after the underlined sentence and finding the compound AND, I went with choice A. But the correct answer is B. Can you please explain the concept when "what is " type sentence constructions are used, what appropriate verb (sing/plural) should we go with.

Thanks

To decide whether a verb should be singular or plural, identify the subject of that verb. The subject is just the person or thing that does the action of the verb. In this problem, we have

"what is much more difficult to determine [are/is] the reason for their decoration, the use to which primitive people put the caves, and the meaning of the magnificently depicted animals"

The subject of the quoted clause is the short clause "what is much more difficult to determine". On the GMAT, always regard a clause as singular when it is the subject of another clause. This means that you should choose "is" rather than "are" for this sentence.

Similarly, "what happens in Vegas" is a clause, and hence a singular subject in the following correct sentences:

"What happens in Vegas stays in Vegas."
"What happens in Vegas is gambling and drinking."

-Jad

Does this ever change if the listed items were in front of the subject?

i.e. ORIGINAL: "...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. "

NEW:
...the reason for their decoration, the use to which primitive people put the caves, and the meaning of the magnificently depicted animals ARE what is much more difficult to determine.

Like the OP, I too was fooled by this. My first instinct was to check the list and I thought it should be ARE. So can the swapped order allow the sentence to use ARE rather than IS?