Unlike the short flights of the shuttle and earlier spacecraft, which carried sufficient power in fuel cells and batteries, a permanently orbiting space station will have to generate its own electricity.

A.      the short flights of the shuttle and earlier spacecraft, which carried sufficient power in fuel cells and batteries

B.       the shuttle and earlier spacecraft, with sufficient enough power in fuel cells and batteries for their short flights

C.      the short flights of the shuttle and earlier spacecraft, which enabled them to carry sufficient enough power in fuel cells and batteries

D.      the shuttle and earlier spacecraft, which were capable of carrying sufficient power in fuel cells and batteries for their short flights

E.       the flights of the shuttle and earlier spacecraft, whose shortness allowed them to carry sufficient power in fuel cells and batteries

答案是D,但D选项里的were为什么用复数?求教大家帮忙,先谢了