英语虚拟语气Could she lend us a helping hand,she would do so.在表示于将

英语虚拟语气
Could she lend us a helping hand,she would do so.
在表示于将来相反时,从句能用could 怎么我记得好像只能用should呢?
If she could lend us a helping hand,she would do so.
If she should lend us a helping hand,she would do so.
Do you think the thief entered through the window?
—No,if he had,I don’t believe,__A_____ broken the living-room’s window.
A.he would havex05x05x05B.he must have
C.he hadx05 D.should he have
这题答案是D 想不通啊,Why?