The question occurred to me ----we were to get the machines

The question occurred to me ____we were to get the machines needed.A.that B.what C.why D.where请问下这道题选什么呢,其它答案为什么错了呢,还有最后一个词need在这怎么理解呢,也是question 的同位语从句，不过就是有点不懂为啥选D，C哪里不对呢