He has the lower molecular mass and higher velocity therefore the curve A refers to He at the same temperature.
Kinetic energy and temperature of a gas
We know that temperature is a measure of the average kinetic energy of the molecules of a gas. The average kinetic energy of gas molecules directly varies with temperature but inversely varies with the molecular mass.
At the same temperature, the velocity of gas molecules depends on the molar mass of the gases. Hence, He has the lower molecular mass and higher velocity therefore the curve A refers to He at the same temperature.
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