( )there when the accident occurred,i would not have believe

选D
这是已经发生了的动作的假设,结构为：
if＋主＋had ＋p.p.,主＋should/would (not)＋have p.p.
这里从句里的if也可以省略,但要用倒装；把助动词提前,但是不提前否定词.即把had提前即可.
Had i not been there when the accident occurred,i would not have believed it.
如果事故发生的时候我不在那里,我就不会相信了.
只有D符合,其他的ABC就排除掉啦.