Correct Answer - C
`because` Circumference of first circle = `2pir = pid_(1) = 36 pi ` cm [given, `d_(1)` = 36 cm]
and circumference of second circle = `pid_(2) = 20 pi cm` [given, `d_(2)` = 20cm]
According to the given condition,
Circumference of circle= Circumference of first circle + Circumference of second circle
`rArr piD = 36pi + 20pi` [where, D is diameter of a circle ]
`rArr` D = 56cm
So, diameter of a circle is 56 cm.
`:.` Required radius of circle = `(56)/(2)=28cm`