Mr. Smith is from England. He teaches French in our school. His English classes are very interesting. He likes us and we like him.
Mr. Smith has two children--Tom and Mary. Tom is seven and his sister Mary is four. Tom goes to school but Mary doesn't. Mr. Smith likes to wear a T-shirt and jeans. Football is his favorite sport. After school we often have a basketball game. Sometimes Mr. Smith watches and plays basketball with us. He plays basketball just for fun .

如图是汽车、摩托车油量表（实际上就是电流表）原理示意图，R′是定值电阻，R是滑动变阻器，则（　　）