Sentence Correction

Only when a product is demanded by sufficiently large numbers of people is mass production feasible.

a)
b) a sufficient amount of people would mass production become
c) sufficient people would mass production become
d) a sufficient amount of people is mass production
e) sufficient amounts of people is mass production

imo: c

numbers of people (in A) and amount(s) of people (in B,D,E) are incorrect.

what is the OA ?

I came down to A/D, IMO A. I might be wrong because I am not quite familiar with the idiomatic structure of number of...../amount of/numbers of.

Can someone give me something concrete about the structure? Greatly appreciate it!

With the avbl options i'll go with D

In A numbers of is wrong, it shoud be number of

imo D

a) --- redundancy can be ruled out easily
b) a sufficient amount of people would mass production become – keep this
c) sufficient people would mass production become – missing number rules this out
d) a sufficient amount of people is mass production – keep this
e) sufficient amounts of people is mass production – amounts is incorrect

There is tossup between B & D. The sentence begins with only when hence would is correct usage so we can rule out D here. B wins!!!!

What is OA??