A gas is contained in a closed rigid tank. An electric resistor in the tank transfers energy to the gas at a constant rate of 1000W. Heat transfer between the gas and the surroundings occurs at a rate of Q=-50t, where Q is in watts, and t is time, in min. a. Plot the time rate of change of energy of the gas for 0 to 20 min, in watts. b. Determine the net change in energy of the gas after 20 min, in kJ. c. If electricity is valued at $0.08 per kW*h, what is the cost of the electrical input to the resistor for 20 min of operation?