Euler's critical buckling load for a column fixed at both ends is

A

$$\frac{2 \pi^2 EI }{L^2}$$

B

$$\frac{\pi^2 EI }{2L^2}$$

C

$$\frac{4 \pi^2 EI }{L^2}$$

D

$$\frac{\pi^2 EI }{4L^2}$$