单项选择题

America will never again have as a nation the spirit of adventure as it ______ before the West was settled.

[A] has
[B] did
[C] was
[D] would
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单项选择题
The fifth generation computers, with artificial intelligence, ______ and perfected now.
[A] developed
[B] have developed
[C] are being developed
[D] will have been developed
单项选择题
Do open the door, ______ you
[A] can't
[B] don't
[C] wouldn't
[D] won't
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