A Large Population of Laboratory Animals Has Been Allowed to Breed

Multiple Choice

A large population of laboratory animals has been allowed to breed randomly for a number of generations. After several generations, 25% of the animals display a recessive trait (aa) , the same percentage as at the beginning of the breeding program.
The rest of the animals show the dominant phenotype, with heterozygotes indistinguishable from the homozygous dominants.
What proportion of the population is probably heterozygous (Aa) for this trait?

A) 0.05
B) 0.25
C) 0.50
D) 0.75