The lively boy suddenly appeared to be quiet, so his mother knew something unusual must h

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A. casually
B. consequently
(答案→)C. curiously
D. constantly
解析：考查副词辨析。Casually 随便的，偶然的consequently 因此，结果  curiously好奇地 constantly持续地
句意为那个活泼的男孩突然看起来出奇地安静，因此他的妈妈知道不寻常的事情发生了。