The strain energy stored in a cantilever beam loaded as shown, will be

A

$$\frac{ P^{2}l^{3}}{4EI}$$

B

$$\frac{ P^{2}l^{3}}{3EI}$$

C

$$\frac{ P^{2}l^{3}}{2EI}$$

D

$$\frac{ P^{2}l^{3}}{6EI}$$