A cylindrical vessel of radius $0.5\; m$ is filled with oil at the rate of $0.25\; \pi m^3/min$. The rate, at which the level of oil is increasing, is :
( A ) $1 \; m/min$
( B ) $1.25 \; m/min$
( C ) $2 \; m/min$
( D ) $5 \; m/min$