The surface area of a balloon of spherical shape being inflated, increases at a constant rate. If initially, the radius of balloon is \( 3 \) units and after \( 5 \) seconds, it becomes \( 7 \) units, then its radius after \( 9 \) seconds is :
\( 9 \)
\( 10 \)
\( 11 \)
\( 12 \)
