A researcher is studying the effects of anabolic steroids on the aggressive behavior of female rats. Twenty-five female rats received daily injections of a placebo, while twenty-five others received daily injections of the steroid. Round-the-clock videotapes of the communal cages of all rats allow all aggressive encounters to be counted and timed. Which of the following is the independent variable?

the aggressive behavior

the communal cages

the type of injection

the placebo

the rats receiving steroids