Amongst the following statements regarding adsorption, those that are valid are :           (a) ΔHbecomes less negative as adsorption proceeds.           (b) On a given adsorbent, ammonia is adsorbed more than nitrogen gas.           (c) On adsorption, the residual force acting along the surface of the adsorbent increases.          (d) With increase in temperature, the equilibrium concentration of adsorbate increases.