Question 9If $a,$ $b,$ $c,$ $d$ are positive real numbers such that $a,$ $b,$ $c,$ $d$ form an increasing arithmetic sequence and $a,$ $b,$ $d$ form a geometric sequence, then $\dfrac{a}{d}$ is$\dfrac{1}{12}$$\dfrac{1}{6}$$\dfrac{1}{4}$$\dfrac{1}{3}$$\dfrac{1}{2}$