In a Genetic Disorder Caused by a Recessive Allele,scientists Cannot

In a genetic disorder caused by a recessive allele,scientists cannot directly count those who carry the allele in a population.Why is this the case?

A) There is usually no discernible difference between carriers and non-carriers.
B) Recessive disease alleles are usually lost from the population.
C) The genes are most likely X-linked.
D) The alleles are recessive and therefore are very rare.
E) Environmental factors will mask the disease symptoms.