Curved surface area of the cylinder=2×π×r×h To calculate height of the cylinder in terms of 'r', we can see that Δ ADC is similar to Δ EFC Therefore,
AB
DC
=
EF
FC
⇒
10
4
=
h
4−r
⇒h=
5
2
(4−r) Therefore, the curved surface area of the cylinder =2×π×r×
5
2
(4−r)=5πr(4−r) Hence, option B is the correct answer.