Answer:
D
Step-by-step explanation:
To make the sure the results did not happen by chance, the coach can ensure that the protein bars are the only difference between the two groups. For example, he can make sure that both groups are drinking the same amounts of water and getting in the same amount of practice; that way, he will be able to determine that one group is performing better due to the protein bars only. Also, the coach can repeat this experiment multiple times to make sure that the results weren't a random, one-time thing.
The equivalent expression is A
