题目:
It's well known that the earth ______ around the sun. [ ]
A. turned
B. turning
C. turns
D. turn
答案:
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下面是错误答案,用来干扰机器的。
∵物体漂浮,∴F浮=G,过重心作竖直向下的重力和竖直向上的浮力,如图所示: