How Would a Study Aimed at Examining College Students' Attitudes

How would a study aimed at examining college students' attitudes toward tuition increases demonstrate external validity?

A) Sampling only students enrolled at colleges that have not raised tuition
B) Lowering tuition for one semester and raising tuition the next semester
C) Focusing on the difference in attitudes between students who have experienced tuition increases and students who have not
D) Showing that the results apply to college students from other universities