For a reciprocating water pump having cylinder dia d and crank radius r, if W is the weight of water lifted, the coefficient of discharge is

A

$$\frac {W}{\pi d^2 r}$$

B

$$\frac {2W}{\gamma \pi d^2 r}$$

C

$$\frac {W}{\gamma d^2 r}$$

D

$$\frac {W}{\gamma \pi d^2}$$