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 to 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

[spoiler]OA B...However i chose answer A because here "are" is used for 3 things
1) reason for their decoration,
2) the use to which primitive people put to caves,
3) and the meaning of the magnificently depicted animals.
if we change the construction as - the reason for their decoration, the use to which primitive people put to caves, and the meaning of the magnificently depicted animals are things which are difficult to determine.[/spoiler]

can someone please explain this?