38. 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) \( (a-b)^{2} \)
(b) \( 2 \sqrt{a^{2}+b^{2}} \)
(c) \( (a+b)^{2} \)
(d) \( 2\left(a^{2}+b^{2}\right) \)
