GMAT Sentence Correction (SC) | Re: The period when the great painted caves at Lascaux and

This question was first question in GMATPrep Mock1.

The period when the great painted caves at Lascaux and Altamira were occupied by Upper Paleolithic peoplehas 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