单项选择题

However, at times this balance in nature is ______ , resulting in a number of possibly un foreseen effects.

A．troubled
B．disturbed
C．confused
D．puzzled
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单项选择题
Criticism and self-criticism is necessary ______ it helps us to find and correct our mistakes.
A．by that
B．at that
C．on that
D．in that
单项选择题
We had to ______ a lot of noise when the children were at home.
A．go in for
B．hold on to
C．put up with
D．keep pace with
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