Sentence Correction

Someone please explain the following.

The period when the great painted caves at Lascaux and Altamira were occupied by Upper Paleolithic peoples has been established by carbon dating, but what is much more difficult to determine are the use to which primitive peoples put the caves, the reason for their deterioration, and the meaning of the magnificently depicted animals.

a) has been established by carbon dating, but what is much more difficult to determine are
b) have been established by carbon dating, but what is much more difficult to determine are
c) have been established by carbon dating, but what is much more difficult to determine is
d) has been established by carbon dating, but what is much more difficult to determine is
e) are established by carbon dating, but that which much more difficult to determine is

OA: D

The period-- is the first subject and singular. So it should be followed by has. Have/are are incorrect here.
but what is much more difficult to determine-- Shows a contrast which cannot be achieved by carbon dating-- singular-- hence use of "is" is correct.

From the above, Eliminate B,C and E for using Have/are
Eliminate A for using are
That leaves us with D-- Correct answer.

Hope it helps.

Anindya Madhudor wrote:Someone please explain the following.

The period when the great painted caves at Lascaux and Altamira were occupied by Upper Paleolithic peoples has been established by carbon dating, but what is much more difficult to determine are the use to which primitive peoples put the caves, the reason for their deterioration, and the meaning of the magnificently depicted animals.

a) has been established by carbon dating, but what is much more difficult to determine are
b) have been established by carbon dating, but what is much more difficult to determine are
c) have been established by carbon dating, but what is much more difficult to determine is
d) has been established by carbon dating, but what is much more difficult to determine is
e) are established by carbon dating, but that which much more difficult to determine is

OA: D

Check for possible errors in the sentence:
1. peoples has been established
HAS is correct here since we are referring to a singular antecedent "THE PERIOD"
2. is much more difficult to determine are
ARE - incorrect plural verb, subject verb agreement mismatch. So it should be IS

Hence the correct option is 'D'

Did anyone consider the possibility of a reversal.

Put the subject ahead of the verb.