A large number of water drops, each of radius $$r$$, combine to have a drop of radius $$R$$. If the surface tension is $$T$$ and mechanical equivalent of heat is $$J$$, the rise in heat energy per unit volume will be:

A

$$\frac{2T}{rJ}$$

B

$$\frac{3T}{rJ}$$

C

$$\frac{2T}{J}\left(\frac{1}{r}-\frac{1}{R}\right)$$

D

$$\frac{3T}{J}\left(\frac{1}{r}-\frac{1}{R}\right)$$