Which among the following statements are correct with respect to adsorption of gases on a solid?

Question

1. The extent of adsorption is equal to $kp^n$ according to Freundlich isotherm.
2. The extent of adsorption is equal to $kp^{1/n}$ according to Freundlich isotherm.
3. The extent of adsorption is equal to $\frac{a+bp}{ap}$ according to Langmuir isotherm.
4. The extent of adsorption is equal to $\frac{ap}{1+bp}$ according to Langmuir isotherm.
5. Freundlich adsorption fails at a low pressure.

(a) 1 and 3
(b) 1 and 4
(c) 2 and 3
(d) 2 and 4