英语试题10The chemical facts---as a by-product of their serach f

英语试题10
The chemical facts---as a by-product of their serach for gold became the basis for modem chemistry.
A that had been accumulated by alchemists
B were accumulated by alchemists
C alchemists accumulating them
D had been accumulated by alchemsits
为什么选A 不是 B C D 错在哪