Hey,

Could someone please explain the difference between has been versus had been?

I'm looking at question #24 in the OG Purple book (page 237) and I'm confused between the answer choices:

The period when the great painted caves at Lazcaux and Altamira were occupied by Upper Paleothilic people has been established by carbon-14 dating, but what is much more difficult to determine are the reason for their decoration, the use of....and ...

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

My assumption is that "has been" is singular and "have been" is plural and given that "period" is singular, I need to pick "B".

Thoughts?