Question 9Real numbers $a,$ $b,$ and $c$ have arithmetic mean $0.$ The arithmetic mean of $a^2,$ $b^2,$ and $c^2$ is $10.$ What is the arithmetic mean of $ab,$ $ac,$ and $bc?$$-5$$-\dfrac{10}{3}$$-\dfrac{10}{9}$$0$$\dfrac{10}{9}$