Under what conditions is \sqrt{a^{2}+b^{2}}=a+b true, where a and b are real numbers?
Answer Choices
A. It is never true.
B. It is true if and only if a b=0.
C. It is true if and only if a+b \geq 0.
D. It is true if and only if a b=0 and a+b \geq 0.
E. It is always true.