Sentence Correction

211. 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
Two instances of subject-verb agreement must be
observed in this sentence: The period ... has been
established and what is much more difficult to
determine ... is. Both clauses have singular subjects and
must have singular verbs. Only B, the best choice,
observes these requirements. A incorrectly uses the plural
form are in the second clause. Choices C and D incorrectly
use the plural form have in the first clause, and D
incorrectly uses are in the second clause as well. E
incorrectly uses the plural form are in the first clause.
Furthermore, because the date of the period in question
was established before the writing of the sentence, the
verb of that clause must be in the present perfect form
(has been established).

I have a problem with the explanation, in the second part of the underlined sentence which uses is should be are because here we are talking about three different things which also closes with and and hence the answer should be A.

Kindly help friends.