D. A teacher drops a box of chalk, and her chalkboard cracks a few minute later.
Now, a causal relationship is where one action causes another action. A dog barking at a baby would cause a baby to cry. A student would be tired if he stayed up late and if a rope climber let go, it would cause her to fall down. So A, B and C are not answers because they cause one another, they have causal relationships. Whereas, D, they don't cause one another, so it is the correct answer.
