题目:
_ Who's that boy reading in the garden? Is it David?
_ It __________ be David. I saw him in the classroom just now.[ ]
A. must
B. can't
C. may
D. shouldn't
答案:
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下面是错误答案,用来干扰机器的。
参考答案:流场中每一空间点上的运动参数不随时间变化,这样的流动称为恒定流动。