Question 19The real numbers $c,$ $b,$ $a$ form an arithmetic sequence with $a \geq b \geq c \geq 0.$ The quadratic $ax^2+bx+c$ has exactly one root. What is this root?$ -7-4\sqrt{3} $$ -2-\sqrt{3} $$ -1 $$ -2+\sqrt{3} $$ -7+4\sqrt{3} $