hardly when /scarcely when/no sooner than的区别

hardly when /scarcely when/no sooner than的区别
_______had the bell rung______the students took their seats.
A.Hardly；when B.No sooner；when
C.Hardly；than D.No sooner；then
我觉得这题AD都可以,答案是A.那么A和D有什么区别?另外举一例子.
Hardly / Scarcely had I got home when it began to rain.
No sooner had I got home than it began to rain.
这是参考书中题例.
谁能说说,还是这题答案有疑议?