Use the fоllоwing tо аnswer the next three questions. The new City of Fаirfаx resident enjoys gardening and realizes she has the perfect backyard to grow vegetables. She decided to do a study to determine which fertilizer (store-bought or homemade) is more effective at increasing tomato plant yield (measured in the number of tomatoes the plant produces). The resident purchases 30 baby tomato plants of the same species and size. She plants them all in her backyard with identical soil, water and sunlight conditions. Then, she randomly divides the 30 plants into two groups. Group S receives the store-bought fertilizer and Group H receives the homemade fertilizer. After 70 days, each tomato plant's yield was measured. What is explanatory variable in this study?