87. Electric energy units are often expressed in "kilowatt- hours." (a) Show that one kilowatt-hour (KWh) is equal to 3.6 x 106 J. (b) If a typical family of four uses electric energy at an average rate of 580 W, how many kWh would their electric bill show for one month, and (c) how many joules would this be? (d) At a cost of $0.12 per KWh, what would their monthly bill be in dollars? Does the monthly bill depend on the rate at which they use the electric energy