How could two waves on a rope interfere so that the rope did not move at all?

if you already have the answer to (a), then just work out how many periods there are in 1 minute, and that's the answer to (c).

r=rho l/a where a is the area, l is the length and rho is resistivity so the greater the area the lesser the resistance thats why the earth pin is thicker bcos current follow the shortest path, so less resistive .