Why is Redi's experiment on spontaneous generation considered a controlled experiment?

Hint: Francesco Redi was an Italian scientist. He designed scientific research to study the spontaneous formation of maggots by placing fresh meat in each of two different jars in \[1668\]. He positively stated that the maggots came from fly eggs and thus aided to refute spontaneous generation.
Francesco Redi did this research to show us that dead maggots or flies would not be able to breed new flies when placed on decomposing meat in a sealed jar, while live maggots or flies would do so. This disproved both the presence of some vital component in once-living creatures, and the requirement of fresh air to breed life.

Complete answer:
There was only one variable that was altered in the research; all the other variables were controlled.
Before Reid's research most scientists felt that life impulsively came from non-living substances. One sample was flies coming out of dead matter. This was thought to be proof that life came from non-life.
Reid places some meat in two jars. He made sure that both tasters of meat were clear of any flies or fly larva. Then one jar was left open so that flies could sit on the meat and lay their eggs on the meat. The second jar was left open to the air but had a screen that banned flies from entering into it and laying their eggs on the meat.

This controlled trial proved that life did not come from nonliving things. The opinion that life comes only from life was established from this research.

One of the utmost glitches with Darwinian evolutionary theory is the explanation of how life came from non-life for purely natural reasons.

Note:
The book is one of the first footsteps in refuting "spontaneous generation"—a theory also recognized as Aristotelian abiogenesis. At the time usual understanding was that maggots arose spontaneously from decomposing meat.
Redi's research on spontaneous generation is considered a controlled experiment because throughout the trial only one variable was altered at a time. But all other variables were kept controlled or unchanged.