A hydrogen atom $$M_H=1.67\times10^{-27}\,\text{kg}$$, initially at rest, emits a photon and goes from the excited state $$n=5$$ to the ground state. The recoil speed of the atom is nearly

A

$$4.2\,\text{m s}^{-1}$$

B

$$4\times10^{-4}\,\text{m s}^{-1}$$

C

$$2\times10^{-2}\,\text{m s}^{-1}$$

D

$$8\times10^2\,\text{m s}^{-1}$$