Let $ABCD$ be a quadrilateral with area 18 with side AB parallel to the side CD and AB=2CD.Let AD be $\perp$ to AB and CD.If a circle is drawn inside the quadrilateral ABCD touching all the sides then its radius is

Question

$(a)\;3\qquad(b)\;2\qquad(c)\;\large\frac{3}{2}$$\qquad(d)\;1$