帮忙看看这道语法题吧，答案是不是错了？

Art museums do not usually think of their collections as capital or consider the interest income that would be generated if a portion of the capital would have been invested in another form.
(A) be generated if a portion of the capital would have been
(B) have been generated if a portion of the capital would have been
(C) be generated if a portion of the capital were
(D) be generated if a portion of the capital was
(E) be generated if a portion of the capital had been

答案是C，可是我查了一下朗文英语词典，无论是从a portion of的角度还是capital的角度，主语都应该是单数吧，所以我觉得D选项比较合适。各位朋友有时间帮忙看一下吧，实在是很困惑，还是有什么其他机关我没看出来呢？

非常感谢。