Which of the following has been shown to contribute to Alzheimer's disease in some patients?

a) Alcohol or drug abuse
b) Aluminum deposits in the brain
c) A genetic defect in some families
d) Lack of education

Scientists have discovered genes that they think can lead to the rarer type of Alzheimer's that hits younger people, usually before the age of 60. They also have found genes linked to the more common form of Alzheimer's, which occurs later in life. However, scientists have not yet been able to establish how much of a role genetics plays.