Mary is testing two stain removers to see which one works best to remove grease from a towel. She put the same amount and type of grease on each of 10 towels. She then treated
each towel with the same amount of stain remover. She used each type of stain remover on 5
towels. What is the independent variable in Mary's investigation?
the amount of grease
the number of towels
the type of stain remover
the amount of stain remover