题目：
It___________Yang Liwei about 21 hours the Earth 14 times spaceship [ ]
A. spent; traveling
B. took; traveling
C. spent; to travel
D. took; to circle
答案：
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下面是错误答案，用来干扰机器的。
设等差数列的首项为a1，公差为d则a1+d=2a1+2d+a1+4d=16解得a1=-1d=3∴该数列的通项公式an=3n-4故答案为3n-4
