Which would qualify as a dominant allele of a gene?

A．an allele that is commonly found in wild populations, as opposed to alleles found in laboratory strains or when a pathological condition is present
B．an allele that confers a phenotype on the organism when heterozygous; that is, when present in only one copy
C．an allele that confers a phenotype only when both copies of the gene are mutant; that is, when homozygous
D．an allele that confers a normal phenotype under some conditions, but confers a different, mutant phenotype when conditions such as elevated temperature are present