问题: 英语单选，大家来帮个忙
The pressure _ causes Americans to be energetic, but it also puts them under
a constant emotional strain.
A) to compete B) competing C) to be competed D) haveing competed
本题为什么选A，不定式表什么意思？
Once environmental damage _, it takes many years for the system to recover.
A) has done B) is to do C) does D) is done
这应该是被动的呀
解答:
A
竞争的压力（不定式作定语）使美国人精力充沛……
D
once是连词，后面要加句子，又因为要用被动，所以排除ABC。
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