A research study compares different laundry detergents to se…

A research study compares different laundry detergents to see which cleans clothes the best. Three white socks are soaked in mud and then left to sit out for one hour. After the hour is up each sock is washed by itself with a different brand of laundry detergent. The socks are then compared to see which is the whitest. What is the independent variable?