A balloon has a mass of 10g in air. The air escapes from it at an uniform rate with a velocity of 5cm/s and the balloon shrinks completely in 2.5 s. What is the average force acting on the balloon.

Question

$\begin{array}{1 1} (a)\; \text{25 dynes}\\ (b)\; \text{2.5 dynes} \\ (c)\; \text{20 dynes} \\ (d)\; \text{2 dynes} \end{array}$