He ______ his homework last week. [ ]
A. didn't
B. didn't do
C. does
D. doesn't
答案:B
在n个H2O分子和m个H2SO4分子中所含氧元素的质量比是______,氢元素的质量比为______.
500kV变电站计算机监控系统与继电保护信息管理系统间的网络通信()
A.直接接入监控系统站控层网络
B.不接入监控系统
C.接入监控系统,但须进行网络间的逻辑隔离