In Langmuir's model of adsorption of a gas on a solid surface:
a. the rate of desorption of adsorbed molecules from the surface does not depend on the surface covered
b. the adsorption at a single site on the surface may involve multiple molecules at the same time
c. the mass of gas striking a given area of surface is proportional to the pressure of the gas
d. the mass of gas striking a given area of surface is independent of the pressure of the gas