我国水能资源最丰富的水系是[ ]
A.长江
B.黄河
C.珠江
D.雅鲁藏布江
答案:A
7.--- How did you find him out ?
---- I _____ his name by chance on a list.
A.came down
B.came about
C.came up
D.came across
简述发电机逆功率运行的处理。