It is generally the case that most great scientists have their ground-breaking ideas when they are in early to middle adulthood, but rarely when they are older. What might explain this fact?

A) The brain shrinks with age.
B) General intelligence declines steeply after middle adulthood.
C) Fluid intelligence peaks in middle adulthood, and then declines in later life.
D) Crystallized intelligence peaks in middle adulthood, and then declines in later life.

Answer: C
Rationale: Crystallized intelligence tends to increase throughout life. Fluid intelligence peaks around early to middle adulthood. Making ground breaking discoveries can require problem solving that requires new ways of thinking, not just acquired knowledge, this is likely the reason scientific breakthroughs decline with age.