In an experiment on the effect of exercise on memory, the independent variable would be which of the following?

Gender of the experimenter

Number of hours of exercise

Gender of the subjects

Recall score on a memory test

Correct me if I am wrong but wouldn't this be a the gender of the experimenter? Since the numbers of hours can change, the gender of the subjects can change, and the recall score on a memory test can also change. Wouldn't that be right.