I would have come sooner ____possible,but I____that your were still waiting for me.

题目

I would have come sooner ____possible,but I____that your were still waiting for me.
A.it were...didn't konw
B.were it ...hadn't know
C.it had been..wouldn't have known
D.had it been...didn't know.
答案是选D.
希望可以解释一下这句话的意思..和为什么选D...

答案

（过去）如果可能的话我当时就会早点来了,但是我那个时候不知道你是不是仍然等待着我
表示过去发生的事情,it had been表示虚拟语气