If $u = \sqrt{a^2 \cos^2 \theta +b^2 \sin^2 \theta} + \sqrt{a^2 \sin^2 \theta + b^2 \cos^2 \theta}$, then the difference between the maximum and minimum values of $u^2$ is given by

Question

( A ) $2\sqrt{a^2+b^2}$
( B ) $2(a^2 + b^2)$
( C ) $(a-b)^2$
( D ) $(a+b)^2$