Three identical small metal spheres A, B, and C are isolated in space. Initially, sphere A carries +12nC, sphere B carries −6nC, and sphere C is neutral. First, spheres A and B are brought into contact and then separated. Next, sphere B is brought into contact with sphere C and then separated. After these two contacts, what is the final net charge on sphere C?
a) +3.0nC
b) −1.5nC
c) +1.5nC
d) +0.75nC