a. A tank holds 1000 liters of pure water. Brine which contains 0.05 kg of salt per liter enters the tank at the rate of 5 liters per minute. The solution is kept thoroughly mixed and is drained from the tank at the rate of 5 liters per minute. If m is the mass of salt in the tank at time t, which of the following options describes the rate of change of the mass of salt in the tank? dm/dt = m/200 Odm/dt = 0.25 - (m/200) Odm/dt = 0.05 - (m/200) Odm/dt = (50 - m )/200