A. Steel is easy to rust in humid air, and contact with acid, alkali or salt will accelerate steel rust, correct;

B. Plastic is an organic synthetic material, which is wrong;

C. When electrolyzing water, dilute sulfuric acid is added to enhance the conductivity of water, not to increase the amount of hydrogen and oxygen. Wrong;

D. In agriculture, Bordeaux solution for preventing and treating plant diseases can be made by mixing lime milk and copper sulfate solution. Correct.

So choose ad.