During an induction procedure, a positively charged rod is held near a neutral metal sphere. Electrons flow through the grounding wire so that 4.0×109 electrons leave the sphere and enter Earth before the ground wire is disconnected. After the rod is finally removed, what is the sphere’s net charge? (The elementary charge is 1.6×10−19C.)
a) +6.4nC
b) −0.64nC
c) +0.64nC
d) +0.64μC