What is the main purpose of controlled variables in an experiment

Which table best shows the impacts resulting from human activity? impact of human activities human activity impact use of manure and other organic fertilizers reduction in population of birds use of pesticides in farms increase in bird population in nearby wetlands impact of human activities human activity impact use of manure and other organic fertilizers reduction in animal population use of pesticides in farms increase in population of native insects and bugs impact of human activities human activity impact use of plastic decrease in marine animal population removal of beach grass increase in pollution of ocean impact of human activities human activity impact use of plastic increase in fish population removal of beach grass decrease in sand erosion