Let a, b, and c be positive integers with a \geq b \geq c such that

\begin{aligned} a^{2}-b^{2}-c^{2}+a b =2011 \text { and } a^{2}+3 b^{2}+3 c^{2}-3 a b-2 a c-2 b c =-1997 . \end{aligned}

What is a ?

**Answer Choices**

A. 249

B. 250

C. 251

D. 252

E. 253