Which of the following experimental observations would suggest that a disease has a genetic basis?
A. The frequency of the disease is less likely in relatives that live apart compared with relatives that live together.
B. The frequency of the disease is unusually high in a small group of genetically related individuals who live in southern Spain.
C. The disease symptoms usually begin around the age of 40.
D. It is more likely that both monozygotic twins will be affected by the disease than will dizygotic twins.