自考《英语（一）》试题练习：介词的用法（8.6）
单选题
[2005.4]In one type of laboratory test all the variables ________one are controlled.
A.by
B.and
C.but
D.or
单选题
[2005.4]In one type of laboratory test all the variables ________one are controlled.
A.by
B.and
C.but
D.or
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