When $U^{238}$ nucleus originally at rest, decays by emitting an alpha particle having a speed $u$, the recoil speed of the residual nucleus is :

Question

( A ) $\frac{4u}{234}$
( B ) $-\frac{4u}{234}$
( C ) $-\frac{4u}{238}$
( D ) $\frac{4u}{238}$