问题
选择题
The students usually stop _______ when the bell for class _______. [ ]
A. talking; rings
B. to talk; ringing
C. talking; ring
D. to talking; rings
答案
答案：A
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The students usually stop _______ when the bell for class _______. [ ]
A. talking; rings
B. to talk; ringing
C. talking; ring
D. to talking; rings
答案：A
的2倍，已知地面附近的重力加速度为g．试求该人造地球卫星运动时的：