Ask him to ring me up if you ------------ him.

Ask him to ring me up if you ____________ him.
A) should see C) .saw
B) would see D) will see
这题为什么选A,个人认为如果A能选,那么B也可以,所以AB排除,D更不能选,应该是C才对呀.
而且,与将来事实相反,也应该是从句用一般过去时啊.