10th grade Geometry Problem in IIT-JEE 2007 Advanced MCQ || Area of quadrilateral | Tangent |circle

This MCQ question was asked in IIT-JEE 2007 from 10th grade Geometry. The question is as following : ABCD is a quadrilateral with area 18 cm^2. AB // CD and AB = 2CD, AD is perpendicular to CD. A circle touches all the four sides of the quadrilateral ABCD. What is the radius of the circle..?? #IITJEE, #JEEMAINS, #JEEADVANCED, #NTSE, #SSC, #NDA, #SSC