The mean of x₁ and x₂ is M₁ and that of x₁, x₂, x₃, x₄ is M₂ then the mean of ax₁,ax₂,x₃/a,x₄/a is
a) (aM₁ + M₂/a)/2
b) M₁ + M₂/2
c) 1/2a[(a² - 1 )M₁ + M₂]
d)1/2a[(a² -1 )M₁ + 2M₂]