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

A

$$2(a^2 + b^2)$$

B

$$2\sqrt{a^2 + b^2}$$

C

$$(a + b)^2$$

D

$$(a - b)^2$$