Recessive Mutations in the Sxl Gene in Drosophila Are Lethal

Recessive mutations in the Sxl gene in Drosophila are lethal in homozygous XX females because

A) without Sxl protein the expression of genes on both X chromosomes is increased.
B) increased Sxl protein signals male-specific development.
C) these mutations are the result of a loss of both X chromosomes.
D) Sxl is required for transcription of male-specific genes.
E) recessive mutations are always lethal in homozygotes.