A:开采地下水
B:节约用水
C:修建水库
D:实行人工降雨
-- Many students don't know what to stress and become worried.
-- I think they'd better ask their teachers for help.
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The People's Republic of China was ______ on October1st,1949.
如图所示,弹簧的上端固定在天花板上,下端连一小球A,球A与球B之间用线相连.A、B的质量均为m,弹簧与线的质量均可不计.开始时它们都处在静止状态.现将A、B间的线突然剪断,在剪断线的瞬间A、B的加速度的大小为( )