Stern developed the concept of an Intelligence Quotient (IQ). If Kiera's mental age (e.g., 10) is higher than their chronological age (e.g., 5), it means that _________.
a. Kiera is gifted or has advanced cognitive abilities
b. Kiera may have a learning disability
c. The IQ test may not accurately measure Kiera's intelligence
d. Kiera's environment and experiences are contributing to her advanced mental age