Which of the following events are correlated but do not necessarily have a causal relationship?
a. A dog snarls and barks at a baby, who immediately starts crying.
b. A student stays up late doing homework, and he is very tired the next morning.
c. A rope climber lets go of her rope, so she quickly falls to the ground.
d. A teacher drops a box of chalk, and her chalkboard cracks a few minutes later.