如图，在研究“电流跟电压的关系”时，设电流表的示数为I，电压表的示数为U．则正确的是（ ）
A．I变大时，
变小U I
B．U变大时，
不变U I
C．I变小时，
变大U I
D．U变小时，U I变小
由电路图可知，R0与R′串联，电压表测R0两端的电压，电流表测电路中的电流．
由I=
可知，U R
=R0，U I
因定值电阻的阻值不变，即与两端的电压和通过的电流无关，
所以，电路中的电流I和电压表的示数U变化时，
不变，故B正确，ACD不正确．U I
故选B．
| 完形填空：从A 、B 、C 三个选项中，选择一个最佳答案填空，使短文意思完整。 | |||
| What should we do to keep healthy? One important rule is to exercise 1 . The Smith family try to exercise every day. Mr. Smith 2 exercise in the morning because he must get to work at seven o'clock. But he runs every evening. He walks a lot, 3 . He goes to school on foot every day, and after school he 4 different sports with his friends. Mrs. Smith goes to a yoga (瑜珈) class 5 . But it wasn't always this 6 . Last year Mr. and Mrs. Smith used to (过去常常) take buses 7 , even to the drugstore two blocks away. They wouldn't walk. So they often go to the drugstore. But the Smiths all 8 better now. They believe they shouldn't be lazy. We 9 exercise every day, but we should try our best to exercise 10 often as possible. | |||
|