Water，which seems so simple and common，is ________ makes life possible.
A．what B．that
C．which D．how
I just wonder ________ that makes him so excited.
A．why it does B．what he does
C．how it is D．what it is
________ 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
