What is the following product? Assume $b \geq 0$.
$\sqrt{b} \cdot \sqrt{b}$
A$b \sqrt{b}$
B$2 \sqrt{b}$
C$b$
D$b^2$
$\sqrt{b} \cdot \sqrt{b}$
A$b \sqrt{b}$
B$2 \sqrt{b}$
C$b$
D$b^2$