Frequency-dependent selection acts to decrease the frequency of which phenotype in a population?

Frequency-dependent selection acts to decrease the frequency of which phenotype in a population?
  a. The least common phenotype
  b. The most common phenotype
  c. The most conspicuous phenotype
  d. The most camouflaged phenotype
  e. The phenotype at an extreme of the normal distribution

Question 2

With respect to the alleles for sickle cell anemia, which genotype(s) is (are) at a disadvantage to persons residing in
  tropical areas of Africa?
  a. heterozygous
  b. homozygous dominant
  c. homozygous recessive
  d. both heterozygous and homozygous dominant
  e. both homozygous dominant and homozygous recessive