A detergent company is trying to find a new stain remover ingredient to put in its detergent. Company managers know that baking soda or bleach will be used. They also know that water temperature, cold and warm, affects the stain removal process. An experiment will compare all combinations of these choices. Four types of stains on the same fabric will be subjected to each treatment and scored for stain removal quality.

Required:
What is the experimental unit?