Two charged spheres X, Y are put in contact with a total charge of +12Q. Another charged sphere, Z, came into contact with X and Y. After electrostatic equilibrium among the three, Y is then separated from both X and Z resulting to the final charge of Y as-6Q. Assuming X, Y, and Z are identical in size, what is the original charge of Z? (A) –18Q (B) -30Q (C) 6Q (D) -85Q