Thomson CAT2003中的一道语法题

Puritan fanatics brought to civil and military affairs a coolness of judgment and mutability of purpose that some writers have thought inconsiistent with their religious zeal, but which was in fact a natural outgrowth of it.

a. but which was in fact a natural outgrowth of it

b. but which were in fact a natural outgrowth of it

c. but which were in fact natural outgrowths of it

d. but it was in fact a natural outgrowth of them

e. which was in fact a natural outgrowth of it

答案：c

为什么啊？我想了半天也不明白（其实对于这道题我就没明白）

恳请nn帮助解惑

大全-975

which = ' a coolness of judgment and mutability of purpose'

所以是 复数 were

it = religious zeal

谢谢joe11的解惑