A particle is projected with a velocity $u$, at an angle $\alpha$, with the horizontal. Time at which its vertical component of velocity becomes half of its net speed at the highest point will be
(A) $\frac{u}{2 g}$
(B) $\frac{u}{2 g}(\sin \alpha-\cos \alpha)$
(C) $\frac{u}{2 g}(2 \cos \alpha-\sin \alpha)$
(D) $\frac{u}{2 g}(2 \sin \alpha-\cos \alpha)$