Which of the following is an example of a beneficial mutation in humans?

• A. A missense mutation in the CFTR gene that leads to cystic fibrosis
• B. A mutation in the lactase (LCT) gene that allows adults to digest lactose, improving nutrition in milk-consuming populations
• C. A nonsense mutation that introduces a premature stop codon in the dystrophin gene, causing Duchenne muscular dystrophy
• D. A trinucleotide repeat expansion in the HTT gene that causes Huntington disease

Answer: (B)