ANS:A - removed by acid pickling.

The oxide layer formed on the surface of non-ferrous metals after annealing is typically removed by acid pickling. Acid pickling involves immersing the metal in a solution of acid, which reacts with and dissolves the oxide layer, leaving behind a clean surface. This process helps improve the appearance and quality of the metal surface, making it suitable for subsequent processing or finishing operations. Hammering or using coarse emery cloth may also be employed in some cases to remove surface oxides, but these methods are less effective and may not provide as uniform or thorough a cleaning as acid pickling. Leaving the oxide layer intact is generally not desirable, as it can interfere with subsequent manufacturing processes and may compromise the quality or performance of the finished product. Therefore, removing the oxide layer through acid pickling is the preferred method for preparing non-ferrous metal surfaces for further processing.