Use the Following Information for Questions

Use the following information for questions:
Suppose you were to read about a study showing that people who sleep less than five hours a night have twice as much risk of a premature death as people who sleep seven or eight hours a night.
-Which of the following is not an example of a possible "confounding variable"in this study?

A) Medications a person is taking.
B) The amount of alcohol a person drinks in the evening.
C) The amount of stress a person is under in their work.
D) The amount of sleep a person gets per night.