4. A tank has capacity 800 gallons. Initially it contains 200 gallons of water with 10 kg of salt in solution. Pure water (i.e. water with no salt in solution) flows into the tank at a rate of 2 gallons per minute. The well-mixed tank water is allowed to leave the tank at the same rate of 2 gallons per minute. Which of the following initial-value problems best models the mass Q(t) of salt (in kg) left In the tank at time t, t > = 0?