In aluminium extraction by Bayer process, alumina is extracted from bauxite by sodium hydroxide at high temperatures and pressures
`Al_(2)O_3(s) + 2OH^(-) rarr 2Al_(2)O_(2)^(-) (aq) + H_(2)O `(l)
Solid impurities such as `Fe_(2)O_3` and `SiO_2` are removed and then `Al(OH)_4^(-)` is reprecipitated
`2Al(OH)_4^(-)` `rarr` `Al_(2)O_3.3H_(2)O`(g) + `2OH^(-)` (aq) In the industrical world :

Options

A. carbon dioxide is added to precipitate the alumina
B. temperature and pressure are dropped and the supersaturated solution seeded
C. Both (a) and (b) are practised
D. the water is evaporated

Answer

C

Explanation

Both (a) and (b) are practised.