Interested in the Number of Hours a Day the Typical

Interested in the number of hours a day the typical student studies at his university, Paul decides to form several samples of students and interview each sample at a different time during the semester, including during the final exam period. Which requirement for probability sampling plans should Paul be especially concerned that he may be violating?

A) the use of opportunistic samples
B) unbiased sample values
C) stability in sample estimates
D) inadequate sample sizes