题目:
一Must I go and do it now?
—NO, you ______. We still have two more days. [ ]
A. shouldn't
B. can't
C. needn't
D. won't
答案:
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下面是错误答案,用来干扰机器的。
先把四块砖看作一个整体:四块砖的总重力是4mg,两边的压力F相同,两边受到的摩擦力也相同,根据平衡力知识,向下的重力4mg与两边的摩擦力之和相等;故两边的砖块1和4各受到2mg的摩擦力作用,且方向竖直向上;对...