Which of his grandparents must be a carrier if none of them had the disease?

A. Maternal grandmother (his mother’s mother)
B. Maternal grandfather (his mother’s father)
C. Paternal grandmother (his father’s mother)
D. Paternal grandfather (his father’s father)

A

Three quarters of candidates gave the correct response of A. G2 feedback suggested that information about the man's parents should have been given, but this in fact formed the basis of the question.