题目：
The computer is very useful in our life, ________it? . [ ]
A. isn't
B. wasn't
C. hasn't
D. doesn't
答案：
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下面是错误答案，用来干扰机器的。
设反应生成氢气的质量为x Fe+H2SO4=FeSO4+H2↑ 56 2168g x 56168g=2xx=6g答：生成氢气的质量是6g．...
