A helium atom, a hydrogen atom and a neutron have mases of` 4.003 u, 1.008 u` and `1.009 u` (unified atomic mass units), respectively. Assuming that hydogen atoms and neutrons can be fuse to from helium, what is the binding energy of a helium nucleus?
A. `2.01 u`
B. `3.031 u`
C. `1.017 u`
D. `0.031u`