Let $$\vec{a},\vec{b}$$ be two vectors, and let $$P,Q$$ and $$R$$ be the points with position vectors $$\vec{a}$$, $$\vec{b}$$ and $$\vec{a}+\vec{b}$$, respectively, with respect to the origin $$O$$. If $$|\vec{a}+\vec{b}|=\sqrt{21}$$, $$|\vec{a}-\vec{b}|=3$$, and $$\vec{a}$$ and $$(\vec{a}-\vec{b})$$ are perpendicular to each other, then the area of the triangle $$OPR$$ is

A

$$\sqrt{3}$$

B

$$\dfrac{\sqrt{3}}{2}$$

C

$$\dfrac{3\sqrt{3}}{2}$$

D

$$\dfrac{3}{2}$$