It was possible that 句型后面是固定用虚拟语气么?

题目

It was possible that 句型后面是固定用虚拟语气么?
At first,I thought it was possible that I ____ it home.
A have left
B could have left
我本来是想选B来着,我记得是固定用虚拟语气,但是我看到前面有个possible,已经
表示可能..后面再有个could也是表示可能,就重复了.就没选...结果错了..
答案就是b...

答案

b表示本来能离开家.
主句说“i thought”意为我以为...也就是说,其实并不是这样.故选b