In aluminium extraction by the Bayer process, alumina is extracted from banxite by sodium hydroxide at height temperatures and pressures
`Al_(2)O_(3)(s) + 2OH^(-) (aq) 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(s) + 2OH^(-)` (aq). In the industrial 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