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

问题补充：

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

A. the one

B. one

C. ones

D. another

答案：

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。