题目

 As we all know, it took scientists more than a century _____ the dream of flying in space.    A. realize                B. realizing           C. having realized    D. to realize 答案： D。此题考查it takes sb some time to do sth句型。有机物 有多种同分异构体，其中能够发生水解反应且能与氯化铁溶液发 生显色反应的同分异构体有（不考虑立体异构）A．6种 B．9种 C．15种 D．19种