If an oxygen meter is calibrated above sea level (in Denver, for example) and used at a lower elevation (for example, Boston), it will overestimate oxygen concentrations because.. a.... as the altitude increases, the amount of gas molecules in the air decreases. b. ... as the altitude decreases, the amount of gas molecules in the air decreases. c. ... as the altitude increases, the amount of gas molecules in the air also increases. d. All of the above