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

Question

( A ) $\frac{h}{\sqrt{mkT}}$
( B ) $\frac{2h}{\sqrt{3mkT}}$
( C ) $2 \frac{h}{\sqrt{mkT}}$
( D ) $\frac{h}{\sqrt{3mkT}}$