The rate of accumulation of salt in the tank is equal to: O the product of the concentration and the volumetric flowrate of the incoming solutions O the product of the concentration and the volumetric flowrate of the incoming solutions plus the product of the concentration and the volumetric flowrate of the outgoing solutions O the product of the concentration and the volumetric flowrate of the incoming solutions minus the product of the concentration and the volumetric flowrate of the outgoing solutions O the product of the concentration and the volumetric flowrate of the outgoing solutions minus the product of the concentration and the volumetric flowrate of the incoming solutions