If the plane $2ax - 3ay + 4az + 6 = 0$ passes through the mid point of the line joining the centres of the spheres $x^2 + y^2 +z^2 + 6x - 8y - 2z = 13$ and $ x^2 + y^2 + z^2 - 10x +4y - 2z = 8 $, than $a$ equals :

Question

( A ) -1
( B ) 2
( C ) 1
( D ) -2