If you want to reduce the likelihood of any preexisting differences between the groups of participants assigned to different conditions in an experiment you should use

Random Assignment

Explanation:

Random Assignment

Is to randomly assign subjects to different treatment groups to reduce the chances of preexisting differences between the subjects having an impact (negative) on the experiment.

sulfur dioxide

explanation: acid rain is caused by a chemical reaction that begins when compounds like sulfur dioxide and nitrogen oxides are released into the air. these substances can rise very high into the atmosphere, where they mix and react with water, oxygen, and other chemicals to form more acidic pollutants, known as acid rain.