An investigator conducts an experiment with four treatment groups. The response variable is growth of a plant during the experiment. She performs an F-test. What is the null hypothesis the researcher is testing with the F-test?

a. All plants in all treatment groups have the same growth during the experiment.
b. All four treatment groups have the same average growth of the plants during the experiment.
c. All four treatment groups have a different average growth of plants during the experiment.
d. At least one treatment group has a different average growth of plants during the experiment, but not all four necessarily have different mean growths.
e. All plants in each treatment group have the same growth during the experiment, but plants in different treatment groups might have different growths.