Chọn D.
\(\eqalign{ & \Delta \overrightarrow p = \overrightarrow {p'} - \overrightarrow p = m\overrightarrow {v'} - m\overrightarrow v \cr & = m\left( { - \overrightarrow v } \right) - m\overrightarrow v = - 2m\overrightarrow v \cr & \Rightarrow \Delta p = - 2mv \cr} \)
