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

1. A
   $\frac{h}{\sqrt{3mkT}}$
2. B
   $\frac{h}{\sqrt{2mkT}}$
3. C
   $\frac{2h}{\sqrt{mkT}}$
4. D
   $\frac{h}{\sqrt{mkT}}$