A consumer group tests different laundry soaps to determine which one is the most effective at removing grass stains. They selected 5 different brands or types of laundry soap to compare. There will be one additional group that received no laundry soap to serve as the negative control. What is the independent variable in this experiment?
O grass stains
O type of laundry soap
O grass stain remaining after washing
O Cannot be determined from this information