Correct Answer - C
Aluminium , a very active metal, reacts rapidly with `O_(2)` form the air to form a surface layer of aluminium oxide, `Al_(2)O_(3)`, that is so thin that it is trasparent. This very tough, hard substance is inert to `O_(2), H_(2)O`, and most other corrosive agents in the environment. In this way objects made of `Al` form their own protective layers and need not be treted further to inhibit corrosion. However, acid rain endangers structural `Al` by dissolving this `Al_(2)O_(3)` (amphoteric oxide) coating.
Similarly `Ti` (which is often used to make sports bikes) does not corrode becuase of a hard, impenetrable layer of `TiO_(2)` that adheres to the surgace and protects themetal form further oxidation.
