Dr. Ferguson is conducting a research study on the effects of a vegetarian diet on women exercisers' body image. After completing her study, Dr. Ferguson is concerned that the improvements in women exercisers' body image may actually be due to the social support they received from the other study participants rather than the vegetarian diet. Dr. Ferguson's concern represents a threat to what type of validity?

a. External validity

b. Construct validity 

c. Internal validity 

d. Logical validity