Which Statement Regarding Partial Chromosomal Deletions or Duplications Is True

Which statement regarding partial chromosomal deletions or duplications is true?

A) They result in either a triple dose of some gene alleles or a single dose of some alleles.
B) They usually result in fewer or milder manifestations than do trisomies or monosomies.
C) The majority of partial chromosomal deletions or duplications result from advanced parental age.
D) General prenatal chromosomal analysis has resulted in fewer children being born with these problems.