A student wonders if adding fertilizer to the soil will make tulips grow faster. The student’s hypothesis states, “If fertilizer is added to the soil, then the tulips will grow faster, because fertilizer adds nutrients to the soil.” Which of the following variables is the independent variable?

The independent variable is the factor the student deliberately changes. Here, the student changes whether fertilizer is added to the soil, so the soil condition—soil with or without fertilizer—is the independent variable. The time it takes the tulips to grow is the outcome being measured, so it is the dependent variable.

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