Behind your house, there's a gorgeous pond filled with frogs. You notice that the frogs behind Mr. Archer's house often have three legs. You start wondering if the fertilizer that Mr. Archer uses for his lawn is causing the deformities in the frogs. You collect 10 frogs and put the special fertilizer in their container (Group A). You put another 10 frogs and place them in a different container with just water (Group B). A week later, 7 out of 10 of the frogs in group A grew only three legs. The frogs in group B grew four legs. What is the independent variable?