AMC 10A 2011 Problem 16

Which of the following is equal to \sqrt{9-6 \sqrt{2}}+\sqrt{9+6 \sqrt{2}} ?

Answer Choices
A. 3 \sqrt{2}
B. 2 \sqrt{6}
C. \dfrac{7 \sqrt{2}}{2}
D. 3 \sqrt{3}
E. 6