Answer:
the answer is "c"
Step-by-step explanation
control variables are any thing that are held constant or limited in a research study. it is a variable that is not of interest to study objectives, but is controlled because it could influence the out comes. so in the research above we need to study the influence of getting too little sleep on human immune. in other word getting too little sleep is independent variable which may have an effect on dependent variable human immune. also human immune response is a value we have to measure to decide whether the independent variable affect the result or not. control variables for the above research are like the food the experimental group and the standard group eat, the bed the in which the experiment is done and so on.