Lactic acid bacteria are used when making kimchi. Lactic acid bacteria ferment to produce lactic acid, which gives the dish a special flavor. At the same time, lactic acid bacteria are anaerobic bacteria and do not require oxygen to decompose organic matter, so the kimchi jar must be covered and used. A circle of water should be used to seal the seal to prevent outside air from entering. Otherwise, if air enters, it will inhibit the fermentation of lactic acid bacteria and affect the quality of the kimchi. At the same time, the lid can also prevent the sour taste of lactic acid produced in the kimchi jar from evaporating, and can Increase the humidity of the air inside the altar. Therefore A is correct.

So the answer is: prevent air from entering the altar and maintain an oxygen-deficient environment in the altar.