Which of the following observations indicate chemisorption rather than physisorption?

1. AEnthalpy of adsorption is $-150\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$
2. BThe amount adsorbed at fixed pressure first rises with temperature from $300$ to $600\mathrm{K}$ and then falls
3. CThe amount adsorbed falls steadily on warming and the enthalpy is $-20\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$
4. DThe gas is fully recovered on reducing the pressure at the same temperature and multilayers are formed