A sealed container with negligible co-efficient of volumetric expansion contains helium (a monatomic gas) when it is heated from $200 \mathrm{~K}$ to $400 \mathrm{~K}$, the average $\mathrm{K} . \mathrm{E}$. of helium atom is
(A) Halved
(B) Doubled
(C) Unchanged
(D) Increased by factor $\sqrt{2}$