The de-Broglie wavelength of a neutron in thermal equilibrium with heavy water at a temperature T (Kelvin) and mass m, is

A

$$\frac {2h}{\sqrt 3mkT}$$

B

$$\frac {2h}{\sqrt mkT}$$

C

$$\frac {h}{\sqrt mkT}$$

D

$$\frac {h}{\sqrt 3mkT}$$