Use the following study to answer the following question(s): A researcher applies varying amounts of fertilizer (0, 2, 4, 8, 10 units) to 50 potted tomato plants. All other variables that may affect the outcome (watering, temperature, sunlight, plant size, etc.) are kept the same from pot to pot. At the end of the growing season, the tomatoes grown on each plant are weighed to determine which fertilizer level produces the largest tomato yield. Which variable is the independent variable?