The mass of a planet is $\frac{1}{10}$ th that of the earth and its diameter is half that of the earth. The acceleration due to gravity on that planet is:

1. A
   $19.6{\text{m s}}^{-2}$
2. B
   $9.8{\text{m s}}^{-2}$
3. C
   $4.9{\text{m s}}^{-2}$
4. D
   $3.92{\text{m s}}^{-2}$