单项选择题

Some of the experiments（）in our textbook were very difficult to carry out.

A. to describe
B. described
C. describing
D. to have described

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单项选择题
Criticism and self-criticism is necessary（）it helps us to correct our mistakes.

A. in that
B. in which
C. unless
D. until
单项选择题
The kids lay face down on the beach, their backs（）to the sun.

A. exposing
B. were exposing
C. exposed
D. having exposed
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