Why is it important to have a clean surface in surface studies?

In Langmuir's model of adsorption of a gas on a solid surface:

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.

Which of the following statements is correct for physisorption?

The charge on $As_2S_3$ sol is due to the adsorption of........

In the adsorption of a gas on a solid,the Freundlich isotherm is obeyed. If the slope of the plot of $\log(\frac{x}{m})$ versus $\log(p)$ is zero,then the extent of adsorption is: