单项选择题

The doctor will not perform the operation______it is absolutely necessary.

A. so
B. if
C. for
D. unless
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单项选择题
Frank's lessons were too hard for him, and he soon fell______the rest of the class.
A. behind
B. down
C. off
D. away from
单项选择题
This question is too hard, it is______my comprehension.
A. below
B. beyond
C. over
D. without
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