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what does an organism's phenotype describe?
the allele combination for a trait
the physical appearance of a trait
the number of alleles on a gene
the number of different traits inherited

The physical appearance of a trait.

Explanation:

Lets look at the definition according to google dictionary, "the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment."

This basically sums up that the phenotype is the physical appearance of a trait while the genotype is the genes and inner process of appearance. So a phenotype is what describes physical characteristics.

its c its is a mixture of gases

temp rises, and expansion