It was not until man had learned how to make iron and steel

It was not until man had learned how to make iron and steel that the construction of various...
It was not until man had learned how to make iron and steel that the construction of various machines________possible.
A.were made
B.made
C.had made
D.was made

D.直到人类学会炼制金属,各种机械的制造才成为可能.制造是单数,而且是被人做.时态发生在制金属之后.

就是这个解释