A, whether at an altitude of 200m or 3000m, the gas in the food bag is only in different positions, and the quality has not changed. This option meets the meaning of the question;

B, from 200m to 3000m above sea level, the atmospheric pressure decreases, so the gas volume in the food bag expands and the pressure decreases. This option does not meet the problem;

C, from 200m to 3000m above sea level, the atmospheric pressure decreases, so the gas volume in the food bag expands and the density decreases. This option does not meet the problem;

D, due to the change of ambient air pressure, the volume of gas in the food bag changes. This option does not meet the problem.

So choose a.