—It's dangerous to swim in this river. .
—Yes, you are right. The government has people not to swim in it.
A:expected
B:allowed
C:warned
D:attracted
树木到了秋天要落叶是因为它们能适应寒冷的环境。
区别脊椎动物和无脊椎动物的主要依据是体内有无( )
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