You Are Investigating the Role of Physical Activity in Heart

You are investigating the role of physical activity in heart disease and suggest that physical activity protects against having a heart attack. While presenting these data to your colleagues, someone asks if you have thought about confounders such as factor X. This factor X could have confounded your interpretation of the data if it:

A) is a factor for some other disease, but not heart disease.
B) is a factor associated with physical activity and heart disease.
C) is a part of the pathway by which physical activity affects heart disease.
D) has caused a lack of follow-up of test subjects.