Let $a, b$, and $c$ be positive integers with $a \geq b \geq c$ such that $a^{2}-b^{2}-c^{2}+a b=2011$ and $a^{2}+3 b^{2}+3 c^{2}-3 a b-2 a c-2 b c=-1997$. What is $a$ ? 
选项为:A:$$249$$  B:$$250$$  C:$$251$$  D:$$252$$  E:$$253$$  请回答其中一个选项.