( arr[0][0] + arr[0][1] ) ( arr[1][0] + arr[1][1] )

##</code><code>##

C[0] = arr[0][0] arr[1][0]
C[1] = arr[0][0] arr[1][1]
C[2] = arr[0][1] arr[1][0]
C[3] = arr[0][1] arr[1][1]