(2分)由铜、银和硝酸银溶液组成的原电池中,负极反应式为 ,正极反应式为 。
(2分)负极:Cu-2e— ="=" Cu2+ 正极:2Ag++2e— ="=" 2Ag
The problem is too difficult for him. He _______ solve it by himself without any help.
A.can't
B.mustn't
C.shouldn't
D.needn't
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