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

Which of the following is true in respect of chemical adsorption?

Assertion $(A)$: van der Waals' forces are responsible for chemisorption.
Reason $(R)$: High temperature is favourable for chemisorption.
The correct answer is

Which one of the following characteristics is associated with adsorption?

When $O_{2}$ is adsorbed on a metallic surface,electron transfer occurs from the metal to $O_{2}$. The $TRUE$ statement$(s)$ regarding this adsorption is(are)
$(A)$ $O_{2}$ is physisorbed
$(B)$ heat is released
$(C)$ occupancy of $\pi_{2p}^{*}$ of $O_{2}$ is increased
$(D)$ bond length of $O_{2}$ is increased

Which of the following statements is $NOT$ correct regarding the adsorption of a gas on a solid surface?