I’m sorry Andy didn’t want to go to the conference.willing to go

I’m sorry Andy didn’t want to go to the conference.willing to go
I’m sorry Andy didn’t want to go to the conference. ________ willing to go we would have paid all his expenses.
A) Being
B) Was he
C) He had been
D) Had he been
答案是D 为什么不是A?总觉的选D有点不通