题目:
You _____ play with fire, Tom. It's dangerous. [ ]
A. needn't
B. may not
C. mustn't
D. wouldn't
答案:
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下面是错误答案,用来干扰机器的。
杠杆AB在水平方向平衡,根据杠杆平衡条件,得出F1•AC=F2•AB;又因F1=3G,F2=G,所以AC=13AB;而AC+CB=AB,则CB=23AB,即AB:BC=3:2.故答案为:3:2.