[求助] a SC question from OG

From the "GMAT Verbal Reviw" (the purple OG) 

16. Declining values for farm equipment and land, the collateral against which farmers borrow to get through the harvest season, is going to force many lenders to tighten or deny credit this spring.

(A)

(B) which farmers use as collateral to borrow against to get through the harvest season, is

(C) the collateral which is borrowed against by farmers to get through the harvest season, is

(D) which farmers use as collateral to borrow against to get through the harvest season, are

(E) the collateral against which farmers borrow to get through the harvest season, are

I'm a little confused by the OG's explanation, which says the subject is " values", and then the plural subject values should take the plural verb are.

I think the subject should be "Declining values for ...", and then the verb should be is. So I choose C, but the standard answer is E.

CAN ANYONE GIVE ME A DETAILED EXPLANATION? 

I REALLY APPRECIATE YOUR HELP. 

thank you very much, i got it.

i found there are many similar examples in OG that the v-ing functioned as a modifier and the following noun is the subject of the sentence, after all, gerund is quite different from v-ing.