What Would Be the Best Way to Distinguish Between Two

What would be the best way to distinguish between two alleles and two genes?

A) Examine the proteins they produce;most genes would produce very similar versions of the same protein,but two alleles would produce very different proteins.
B) Examine the proteins they produce;a gene produces one protein,and an allele produces two different proteins.
C) Examine their DNA;the DNA sequences of two different alleles would be more similar to each other than the sequences of two different genes.
D) You can't distinguish between them;there's no actual difference between alleles and genes.
E) Determine their chromosomal location;alleles will always be on different chromosomes,but genes will always be on different copies of the same chromosome.