Erik Is Retired, but He Has Maintained a Very Active

Erik is retired, but he has maintained a very active schedule. He volunteers for a local non-profit organization, plays cards with his friends twice each week, and he does a daily crossword puzzle. He also takes his neighbour's dog for a brisk walk at lunchtime. Which of the following is less likely to develop as Erik ages, as a result of his lifestyle and activities?

A) crystallized intelligence
B) Alzheimer's disease
C) empty nest syndrome
D) Parkinson's disease