某一学生在做四选一的选择题时（只有一个正确选项）可以确定B选项不合题意，其它的都不能确定哪个选项符合题意，于是就猜了一个选项，则这位同学猜对正确的选项的概率是______．

It was nearly an hour _____________ the sleeping pill _____________ effect.

A．when; took

B．before; took

C．since; takes

D．before; would take