把阻值为R的金属丝拉长，它的电阻值将______；把阻值为R的金属丝对折，它的电阻值将______．（均选填“变大”、“不变”或“变小”）
根据影响电阻大小因素可知，当材料、横截面积一定时，导体越长，电阻越大，当材料、长度一定时，导体横截面积越小，电阻越大．
当把金属丝拉长时，长度变长导致电阻变大，同时横截面积变小也导致电阻变大；
当把金属丝对折时，长度变短导致电阻变小，同时横截面积变小也导致电阻变小；
故答案为：变大；变小．
| 阅读理解。 |
| When you are in England, you must be very careful in the street because the traffic drives on the left. You must look at the right first and then the left, then you can cross the street. If the traffic lights are red, the traffic must stop. Then the people on foot can cross the road. If the traffic lights are green, the traffic can go. People on foot mustn't cross. When you go by bus in England, you have to be careful, too. Always remember the traffic moves on the left. Have a look first, or you will go the wrong way. In many English cities, there are big buses with two floors. You can sit on the second floors. From there you can see the city very well. It's very interesting. |
| 1. When you are in England, you must remember that ______. |
[ ] |
| A. there are not any traffic lights. B. the traffic drives on the left. C. the traffic drives on the right. D. there are many cars on the road. |
| 2. Before you cross a street in Shanghai, you must look at ______ first and then ______. |
[ ] |
| A. the left, the right B. the right, the left C. the front, the black D. the back, the front |
| 3. Why must you be careful when you go by bus in England? |
[ ] |
| A. Because there are too many cars. B. Because you may go the wrong way. C. Because the traffic lights are different from ours. D. Because there are too many people. |
| 4. It's interesting to ______ in England. |
[ ] |
| A. look at the people in the street. B. cross the street. C. sit on the second floor of a big bus and see the city. D. look at the traffic lights. |
| 5. Which is the best title of this passage? |
[ ] |
| A. Traveling in England. B. Buses in England. C. Traffic in England. D. Traffic lights in England. |