Which statement best describes dosage compensation mechanisms in mammals and Drosophila?

• A. Mammals upregulate the single X in males via the MSL complex; Drosophila inactivate one X.
• B. Mammals inactivate one X via XIST-mediated silencing; Drosophila upregulates the single X in males via the MSL complex.
• C. Dosage compensation applies only to autosomes.
• D. Dosage compensation always doubles X expression in both sexes.

Answer: (B)