Concept:A mixture has variable composition, so a fixed proportion by mass suggests a compound, not a mixture.
Explanation:Air is a mixture because its constituents, such as
N2,
O2, and
CO2, are not chemically combined.
Mixtures do not have a fixed proportion by mass; the composition of air varies with location and conditions.
So, the statement that air constituents are in a fixed proportion by mass does not support air being a mixture.
The other options, lacking a chemical formula and being separable by physical means, do support air being a mixture.
Answer:A. the constituents of air are in a fixed proportion by mass