A right circular cylinder is partially filled with water. Two iron spherical balls are completely immersed in the water so that the height of the water in the cylinder rises by 4 cm. If the radius of one ball is half of the other and the diameter of the cylinder is 18 cm, then the radii of the spherical balls are

A

6 cm and 12 cm

B

4 cm and 8 cm

C

3 cm and 6 cm

D

2 cm and 4 cm