A researcher is interested in the effect of fertilizer on the numbers of flowers produced. She sets up an experiment with 4 groups of identical rose bushes. Group 1 receives 1 ounce of fertilizer, Group 2 receives 2 ounces, Group 3 receives 3 ounces and Group 4 receives no fertlizer. The plants receive the same amount of water, sunlight and humidity. She counts the number of flowers on each plant at the end of one month. What is the independent variable in the study?
a. number of flowers
b. height of plant
c. amount of water
d. amount of fertilizer