13.一质点沿x轴正向运动,其速度为 =alpha sqrt (x) 式中a为-|||-正的常量,已知 t=0 时, x=0 。质点的速度随时间变-|||-化的规律是() ()-|||-A at-|||-B dfrac ({alpha )^2}(2)-|||-C dfrac (alpha )(2)t-|||-D dfrac ({alpha )^2}(4)t