Question

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 (C) have been established by carbon-14 dating，but what is much more difficult to determine is (D) have been established by carbon-14 dating，but what is much more difficult to determine are (E) are established by carbon-14 dating，but that which is much more difficult to determine is

Answer 1

The correct option is option B which is has been established by carbon-14 dating，but what is much more difficult to determine is.

Step-by-step explanation:

As both, the clauses have singular subjects and thus require both of the verbs to be singular. This characteristic is only evident in option B. For all the other options, the subject-verb argument is not in sync.