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