If Our Ancestors' Mental Abilities Increased in Parallel with an Increase

If our ancestors' mental abilities increased in parallel with an increase in brain size, why has the size of our brain not continued increasing?

A) this evolutionary mechanism is still at work; it just evolves more slowly than previously thought
B) the human skull is currently as large as the exoskeleton will support
C) factors other than the brain also determine human capabilities
D) although humans continue to evolve, language, memory, and attention are fully developed