Which formula defines the sequence \small f\left(1\right)=2, \small f\left(2\right)=6, \small f\left(3\right)=10, \small f\left(4\right)=14, \small f\left(5\right)=18? a \small f\left(1\right)=6, and \small f\left(n\right)=4+f\left(n-1\right) for \small n\ge2 b \small f\left(1\right)=2, and \small f\left(n\right)=2+f\left(n-1\right) for \small n\ge2 c \small f\left(1\right)=2, and \small f\left(n\right)=6+f\left(n-1\right) for \small n\ge2 d \small f\left(1\right)=2, and \small f\left(n\right)=4+f\left(n-1\right) for \small n\ge2