physics mc plz help

What statement B means is that there is only horizontal velocity with no vertical velocity.

This is not always true, except under the situation that R is the highest point on the rebound path.

不知道你會否看意見欄這裏？
我可以解釋給你知：

沒有說 B 一定錯，但有可能不對。
例如舉個例，當 there is no energy loss 的情況，那麼明顯地你要等待 horizontal 再距離 Q 的右方 0.4 m 時才會使 ball 的 motion 只有 horizontal（即 vertically stationary），因此 B 有可能不對，但也有可能正確（例如當有 energy loss 於吁某特別程度時）。

因此，題目問的 must be true 只可以答 C。

2015-06-29 22:18:58 補充：
Energy
= KE + PE
= ½ m v² + mgh
= ½ m ( v_h² + v_v² ) + mgh　v_h 指 horizontal velocity, v_v 指 vertical velocity
= ½ m v_h² + ½ m v_v² + mgh

第一個term不會變，所以當 h 最大時才會使 v_v = 0。

2015-06-30 00:04:38 補充：
horizontal velocity independent of vertical velocity
> Yes

v_h is a constant

v_v is subject to gravitational acceleration

2015-06-30 00:06:19 補充：
Horizontally, speed = 1.2 m/s

Time = Distance/Speed = 0.6/1.2 = 0.5 s

isn't horizontal velocity independent of vertical velocity?
If without horizontal velocity, how C stands?
sorry, i can't understand T_T