In the Mond process, impure nickel is purified because:

1. A$\mathrm{Ni}(\mathrm{CO}){}_4$ forms at low temperature and decomposes at a higher temperature to give pure Ni
2. B$\mathrm{Ni}$ is converted to a soluble cyanide complex and then reduced
3. C$\mathrm{Ni}(\mathrm{CO}){}_4$ forms at high temperature and decomposes at low temperature
4. Dall metals in the impure sample form volatile carbonyls that condense differently