A filtration process operates first at a constant flow rate and then at a constant pressure difference until the pressure difference reaches 500 kN/m2. If the total time taken for this filtration process is 4800 seconds and 2/3 of the total filtrate is obtained at constant pressure difference, calculate the time taken at constant flow rate? a) 1440 seconds b) 960 seconds c) 1920 seconds d) 720 seconds e) 2880 seconds