Four mass less spring whose force constant are `2k , 2k, k` and `2k` respectively are attached to a mass `M` kept on a friction less plate (as shown in figure) if the mass `M` is displaced in the horizontal direction then the frequency of oscillation of the system is
A. `(1)/(2pi) sqrt((k)/(4M))`
B. `(1)/(2pi) sqrt((k)/(7M))`
C. `(1)/(2pi) sqrt((4k)/(M))`
D. `(1)/(2pi) sqrt((7k)/(M))`