A university conducts a series of surveys in which subjects are drawn from first year and graduating students and finds that graduating students demonstrate much higher rates of approval for the university (in terms of whether they think the classes were relevant, the instructors were competent, etc.). The university then claims that more experience with the university leads to higher approval. What source of internal invalidity has the university not considered?

A. selection bias
B. mortality
C. noncomparable groups
D. regression

A. selection bias