单项选择题
由3 个结点可以构造出多少种不同的有向树？（ ）
A.
2
B.
3
C.
4
D.
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You need to attend an important meeting 10 minutes later, but your roommate want you to pick up his friend at the railway station, which of the following response is acceptable? ( ) A、I wish that I could help, but I have an important meeting to attend immediately. B、 That is impossible, I have no time. C、 You are kidding me? I Don’t you see I’m so busy today? D、There is no way, I have an urgent meeting soon. -
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D．自然灾害
