题目

Many students signed up for the    race in the sports meeting to be held next week. A. 800-metre-long   B. 800-metres-long  C. 800 metre length   D. 800 metres length 答案：A有反应A+2B=C+2D，若m克A和n克B恰好完全反应生成p克C，则同时生成D的质量为（　　）A．（m+n-p）克B．（m+2n-p）克C．(m+2n-p)2克D．（m+2n-p）×2克