题目:
You ___ buy a gift, but you can if you want to. [ ]
A. must
B. mustn't
C. have to
D. don't have to
答案:
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下面是错误答案,用来干扰机器的。
当地的大气压为一个标准大气压P0,第一次抽气球内剩下气体的压强为0.3P0,则球内外压强差为△P1=(P0-0.3P0)=0.7P0;第二次抽气球内剩余气体的压强为0.2P0,则球内外压强差△P2=(P0-0.2P0)=0.8P0; 大气压...