题目

郑和下西洋不能持续的根源是                                       （    ）     A．不计经济效益                 B．明朝国力衰退     C．亚非国家抵制                 D．倭寇侵扰沿海   答案：A Many Chinese students think science subjects are ________ foreign languages． [　　] A． more difficult as B． less difficult than C． much difficult than D． so difficult as