what clause

[MaggieW980]

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 I found out that there are three things to determine： the reason， the use，and the meaning.
Also, here a correct sentence from the Internet: What his father left him are a few books.
I am confused by the use of what clause.

[RonPurewal]

please search the forum before posting. thank you.

https://www.manhattanprep.com/gmat/foru ... 31636.html

https://www.manhattanprep.com/gmat/foru ... t7994.html