Why is only one main factor changed in a fair experiment?

Correct answer: To keep the cause and effect relationship clear

A fair experiment is designed to test whether one factor causes a change in another factor. If several important factors are changed at the same time, the result cannot be linked confidently to any one of them. Changing only one main factor while keeping other relevant conditions constant makes the comparison meaningful. The changed factor is treated as the possible cause, and the measured response is then compared. Therefore option B is correct.

For example, if studying how temperature affects evaporation, the temperature may be changed, but the water volume, container, and observation time should remain the same. Changing the volume and temperature together would make the cause unclear. The purpose is not to make the experiment longer, reduce data, or save instruments. Repeated trials and careful measurements may still be needed, but controlling variables is what keeps the cause-and-effect relationship clear.