What seems to be a simple idea always turns out that is hard to put into practiceA. the

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A. the one
(答案→)B. one
C. ones
D. another
解析：句意：看似简单的想法结果总是成为一个很难实施的想法。这里需要代词代替an idea，用one可以泛指可数名词的单数，the one是特指可数名词单数，ones是泛指可数名词复数，another“另一个”，所以选B。