问题
选择题
I often help her ________ English. [ ]
A. study
B. studies
C. studied
D. studying
答案
答案：A
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I often help her ________ English. [ ]
A. study
B. studies
C. studied
D. studying
答案：A
=28.5m高的阳台掉下，下落过程中空气阻力约为本身重力的0.4倍；在妞妞开始掉下时，吴菊萍立刻从静止开始匀加速奔跑水平距离S=9m到达楼下，张开双臂在距地面高度为
=1.5m处接住妞妞，缓冲到地面时速度恰好为零，缓冲过程中的空气阻力不计。g=10
.求：