(2) Experiment B is a comparative experiment to explore the conditions of nail rust. The purpose of the experiment is to isolate oxygen and explore whether nails rust when isolated from air (oxygen). So the answer is: explore whether nails rust when they are isolated from air (oxygen);

(3) The rust condition of iron products is that they are in direct contact with air and water at the same time, so the rust prevention effect can be achieved by painting, covering paint or plating a layer of corrosion-resistant metal on the surface of iron products. So the answer is: iron products are in direct contact with air and water at the same time; Oiling the surface of iron products;

(4) The effective methods to protect metals are:

1, a large number of metals are corroded and wasted every year, so it is necessary to prevent metal corrosion;

2. Full recycling;

3. Control resources and concentrate on development to avoid waste of metal raw materials caused by limited development conditions;

4. Actively develop and find solutions to replace high-priced metals with low-priced metal alloys or composite materials.

Therefore, the answer is: prevent metal corrosion; Recycling scrap metal.