composite function involving word problem

please help, i'm not sure if i'm on the right track.

**A spherical hailstone grows in a cloud. The hailstone maintains a spherical shape while its radius increases at a rate of 0.2 mm/min. **

**a) Express the radius, r, in millimeters, of the hailstone, as a function of the time, t, in minutes.**

*i came up with the answer, *

*r(t) = 0.2t*

*but i'm not sure if its right*

**b) Express the volume, V, in cubic millimeters, of the hailstone, in terms of r.**

*i got,*

*V(r) = $\displaystyle \frac{4}{3}{\pi}(0.2t)^3$*

**c) Determine (Vor)(t)**

*Vor(t) = $\displaystyle \frac{4}{3}{\pi}(0.2t)^3$*

*not sure if thats right either?*

**d) What is the volume of the hailstone 1 h after it begins to form?**