蹦极运动员从高处跳下，弹性绳被拉展前做自由落体运动，被拉展后在弹性绳的缓冲作用下，下落到一定高度后速度减为零。下列说法中正确的是( )。
A．弹性绳拉展前运动员处于失重状态，弹性绳拉展后运动员处于超重状态
B．弹性绳拉展后运动员先处于失重状态，后处于超重状态
C．弹性绳拉展后运动员先处于超重状态，后处于失重状态
D．运动员一直处于失重状态
参考答案：B
解析： 运动员跳下的初始阶段是自由落体运动，处于失重状态。当弹性绳被拉展后，弹性绳子开始对运动员具有向上的拉力，且不断增大，但在起始阶段拉力小于重力，因而运动员此时仍然向下做加速运动，只是加速度在逐渐减小。故选B。
| 完形填空。 |
| Mr. King is my father's friend. He has some 1 trees in his garden. He 2 them carefully. Now 3 many apples on the trees and Mr. King is very happy. One afternoon when he looked out of the window, he saw a boy on 4 his apple trees. He was eating a big apple! Mr. King 5 the window and shouted (大喊) "Hey, 6 are you doing there? 7 and get away, or (否则) I'll go to see your 8 !" The boy was not afraid of Mr. King 9 all. "Dad," he looked up and shouted, "this man 10 to see you!" |
| ( )1. A. orange ( )2. A. takes care ( )3. A. have ( )4. A. a ( )5. A. opened ( )6. A. how ( )7. A. Come up ( )8. A. sister ( )9. A. in ( )10. A. wants | B. apple B. looks B. has B. one B. turned B. which B. Come over B. friend B. for B. thinks | C. banana C. looks after C. there is C. one of C. closed C. what C. Come C. father C. at C. asks for | D. pear D. watches D. there are D. any of D. turned off D. where D. Come down D. teacher D. with D. needs |