单项选择题

The role of trees is to __________.

A．loose soil
B．keep soil in position
C．harden soil
D．Both A and B
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单项选择题
A．brunchB．breakfastC．lunchD．supper
A．brunch
B．breakfast
C．lunch
D．supper
单项选择题
A．afterB．beforeC．whenD．as
A．after
B．before
C．when
D．as
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