________ caused the accident is still a complete mystery.
A．What B．That C．How D．Where
I’m afraid he’s more of a talker than a doer，which is ________ he never finishes anything. (2011·山东，26)
A．that B．when
C．where D．why
Scientists study ________ human brains work to make computers.(2012·四川，17)
A．when B．how
C．that D．whether
It suddenly occurred to him________he had left his keys in the office.(2012·江西，25)
A．whether B．where
C．which D．that
Jerry did not regret giving the comment but felt ________ he could have expressed it differently. (2012·北京，24)
A．why B．how
C．that D．whether
As many as five courses are provided，and you are free to choose ________ suits you best. (2012·陕西，20)
A．whatever B．whichever
C．whenever D．wherever
