Busy_he was, he tried his best to help us. "
A. as
B. when
C. since
D. for
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After the war, a new school building was put up__there had once been a theatre.
A. as
B. where
C. if
D. when
Don't be discouraged__you have fallen behind others.
A. whether
B. as if
C. even if
D. however
More people will eat out in a restaurant_they do today.
A. as
B. than
C. where
D. though
对于一个气固相催化反应过程,下列属于动力学因素的是_______
A. 气膜扩散;
B. 孔内扩散;
C. 表面反应