A Dominant Trait,Huntington Disease,causes Severe Neural/brain Damage at Approximately Age

A dominant trait,Huntington disease,causes severe neural/brain damage at approximately age 40.The gene that causes this disease is not located on the sex chromosomes.A female whose mother has Huntington disease marries a male whose parents are normal.It is not known if the female has the disease.Assuming the female's mother was a heterozygote,and her father was normal,what is the probability that their firstborn will inherit the gene that causes Huntington disease?

A) 25%
B) 50%
C) 75%
D) 100%
E) 0%