A filtration process operates first at a constant flow rate and then at a constant pressure difference until the pressure difference reaches 500 kN/m. 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 second b]960 second c]1920 second d]720 second e)2880 second