20. In trapezium $P Q R S, S R=P Q=25 \mathrm{~cm}$ and $S P$ is parallel to $R Q$. All four sides of $P Q R S$ are tangent to a circle with centre $C$. The area of the trapezium is $600 \mathrm{~cm}^{2}$. What is the radius of the circle?
A $7.5 \mathrm{~cm}$
B $8 \mathrm{~cm}$
C $9 \mathrm{~cm}$
D $10 \mathrm{~cm}$
E $12 \mathrm{~cm}$