To test that a fertilizer makes flowers produce more blooms, a student tests 6 plants of equal size that all get equal water and sunlight. For 3 of the plants she uses the fertilizer and 3 plants of the plants she does not use the fertilizer. They then count the number of blooms on all the plants for 6 weeks. What is the independent variable?
a. the number of plants in the control group
b. the amount of water
c. the number of flowers
d. the fertilizer