If natural selection is real, why do lethal recessive disorders still exist in populations?

If natural selection is real, why do lethal recessive disorders still exist in populations?
  A. Recessive disorders most frequently do not manifest until later in life, after the individual
  has reproduced.
  B. Recessive alleles remain hidden in carriers who are not affected by the disorder.
  C. There may be an advantage to individuals who are heterozygous.
  D. Recessive disorders most frequently do not manifest until later in life, after the individual
  has reproduced; and recessive alleles remain hidden in carriers who are not affected by the
  disorder.
  E. Recessive alleles remain hidden in carriers who are not affected by the disorder; and there
  may be an advantage to individuals who are heterozygous.

Question 2

The relatively high percentage of individuals in West Africa who are heterozygous for the sickle cell
  trait is an example of _______________.
  A. natural selection D. heterozygote disadvantage
  B. genetic drift E. more than one of these
  C. spontaneous mutational spread