The process ____ water is changed into ice is called freezing.
A. by which
B. to which
C. at which
D. for which
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He tried to quit smoking, ____ he found impossible.
A. as
B. where
C. which
D. when
The reason ____ I am calling you is to tell you a meeting on tomorrow morning.
A. because
B. why
C. as
D. which