C. 28 cm
Diameter of first circle = d1 = 36 cm
Diameter of second circle = d2 = 20 cm
∴ Circumference of first circle = πd1 = 36π cm
Circumference of second circle = πd2 = 20π cm
Now, we are given that,
Circumference of circle = Circumference of first circle + Circumference of second circle
πD = πd1 + πd2
⇒ πD = 36π + 20π
⇒ πD = 56π
⇒ D = 56
⇒ Radius = 56/2 = 28 cm