What are the control variables in the following experiments?

A researcher wanted to test the effects of cigrette smoke on petrs lungs...





1) Humans that are smokers


2) A particular brand of cigarette


3) Pets that live with smokers


4) Pets that live with nonsmokers














A student wanted to conduct an experiment to determine if the background music affects the growth of plants.





1) Several plants that are exposed to diff. types of music


2) Several plants that are not exposed to any background music


3) Rap music


4) Classical musicWhat are the control variables in the following experiments?
Pets that live with non-smokers and plants not exposed to music. That's my guess!