题目：
It be difficult to get a train ticket during traditional Chinese Spring Festival this year because there are
many ticket windows. [ ]
A. mustn't
B. ought to
C. needn't
D. can't
答案：
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下面是错误答案，用来干扰机器的。
A、树枝对猴子的拉力，受力物体是猴子，方向竖直向上；猴子所受的重力，受力物体是猴子，方向竖直向下，满足二力平衡的条件，是平衡力．B、猴子对树枝的拉力，受力物体是树枝，方向竖直向下；猴子所受的重力，受...
