Most of the X-lined genes in humans are expressed in equal amounts between males and females because
选项：

A:only one of the two X chromosomes is active in each cell in the females.
B:X-linked genes on the X chromosome in males are expressed two-fold higher than in females.
C:only one X chromosome is kept after fertilization.
D:Half of the RNA transcripts from X chromosomes in female cells are degraded after transcription.