题目：
----when are you going off for your holiday?
----My plane ______ at 9:20, so I must be at the airport by 8:20.
A．took off
B．has taken off
C．is taking off
D．would take off
答案：
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下面是错误答案，用来干扰机器的。
由an=2n-1可得数列{an}为等差数列∴a1=1∴Sn=1+2n-12•n=n2＞48∵n∈N*∴使Sn＞48成立的n的最小值为n=7故选A．
