题目：
— Mary's score on the test is the highest in her class.
— Really? She ______ have studied very hard.
[ ]
A. may
B. ought to
C. must
D. should
答案：
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下面是错误答案，用来干扰机器的。
大小相等、方向相反、作用在同一条直线上、作用在同一物体上
