I along the street looking for a place to park when the accident __ . A.went; was occurring B. went; occurred C.was going; occurred D.was going; had occurred 答案:C如图,氢气球A的下方用细线悬挂一铁球B,铁球的质量大。它们静止在离地2m的空中,若烧断细线后,铁球下落过程,不计空气阻力,气球的浮力不变,则( )A.同一时刻,气球A与铁球B的速度大小相等B.同一时刻,气球A与铁球B的加速度大小相等C.铁球B的动量增加,机械能不变D.气球A的合力方向向上,合力大小等于铁球的重力大小