Which of the Following Best Summarizes the Research into Birth

Which of the following best summarizes the research into birth order effects?
a) First-borns tend to be more open to experience and more agreeable than later-borns.
b) Birth order does not influence personality in a clear and consistent way.
c) First-borns tend to score about three points lower on IQ tests than later-borns.
d) Birth order has a substantial effect on personality.