转速、电流双闭环直流调速系统中，电流调节器始终饱和。( )

Acid rain is found to have affected vast areas of the Northern Hemisphere.
B. A region without industry can still be affected by acid rain.
C. Power plants, automobiles and factories are known to be the major source of acid rain.
D. Terrestrial vegetation is entirely free from acid rain.

A. Because many old people sell their houses after their children leave.
Because many old houses in the bad part of the town are not inhabited.
C. Because many young people have moved into comfortable apartments.
D. Because many people have quit their old houses to build new ones.

A. They can divert the rivers.
B. They can fix more problems than they create.
C. They are built to irrigate farmland.
D. They may have brought on ecological crisis.