A woman comes for genetic counseling because of a family history of ""muscle disease."" She has two teenaged sons who appear healthy. Her 48yearold maternal uncle is barely ambulatory. Her sister's 17yearold son has muscle weakness and a serum creatine kinase level greater than 5000 U/L. No other family members are known to be affected. The woman's two sons have creatine kinase testing, and they both have levels less than 150 U/L. What is the woman's risk to be a carrier for the condition in her family?

A. 10%
B. 20%
C. 40%
D. 50%