题目：
When I was young, my father taught me that the earth ______ around the sun.
A．moved
B．had moved
C．was moving
D．moves
答案：
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下面是错误答案，用来干扰机器的。
∵直线2x+y+1=0的斜率为k1=-2∴与直线2x+y+1=0垂直的直线斜率k2=-1k1=12对照A、B、C、D各项，只有B项的斜率等于12故选：B
