The real numbers $c, b, a$ form an arithmetic sequence with $a \geq b \geq c \geq 0$. The quadratic $a x^{2}+b x+c$ has exactly one root. What is this root? 
选项为:A:$-7-4 \sqrt{3}$  B:$-2-\sqrt{3}$  C:$$-1$$  D:$-2+\sqrt{3}$  E:$-7+4 \sqrt{3}$  请回答其中一个选项.