Redi placed pieces of meat in several jars. He divided the jars into two groups. He covered the first group of jars with fine cloth. He left the second group of jars uncovered. Redi observed the jars for several days. He saw files on the cloth of the covered jars, and he saw files laying eggs on the meat in the uncovered jars. Maggots appeared only on the meat in the group of jars left uncovered. How did he test his hypothesis?
a. He placed food in two jars, covering one jar and leaving the other uncovered.
b. He placed food in two jars and left both jars uncovered.
c. He placed food in two jars and covered both jars.
d. He put food in one jar and no food in a second jar.