Amongst the following statements regarding adsorption,those that are valid are :
$(a)$ $\Delta H$ becomes 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.

• A
  $(b)$ and $(c)$
• B
  $(a)$ and $(b)$
• C
  $(d)$ and $(a)$
• D
  $(c)$ and $(d)$