问题
选择题
When he left the room, he ________ the door key to me. [ ]
A. handed in
B. handed on
C. handed out
D. handed over
答案
答案：D
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When he left the room, he ________ the door key to me. [ ]
A. handed in
B. handed on
C. handed out
D. handed over
答案：D
位于地球赤道上的某观察者在天黑4小时后，观察者在其正上方仍然可观察到一颗绕地球做匀速圆周运动的人造地球卫星。设地球半径为R，下表列出卫星在不同轨道上飞行速度v大小：
| 轨道半径r | R | 1.5R | 2R | 2.5R | 3R |
| v(km/s) | 7.9 | 6.5 | 5.6 | 5.1 | 4.6 |
A．5.6km/s≤v<7.9km/s B．5.1km/s≤v<6.5km/s C．v=5.1km/s D．v<5.6km/s