Final answer:
The chain used as a class reward system takes 9 links to reach the ground in the book 'Because of Mr. Terupt'.
Step-by-step explanation:
The book 'Because of Mr. Terupt' uses a chain as a class reward system. In the story, it is mentioned that the chain takes 9 links for it to reach the ground. Each time the class earns a reward, they add a link to the chain, until it finally touches the ground. So, it takes 9 links for the chain to reach the ground.