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.