Why Is a Large Refusal Rate a Barrier to Widespread

Why is a large refusal rate a barrier to widespread generalization?

A) If a study has a refusal rate higher than 30%, it is no longer considered research.b.If researchers cannot recruit subjects into their research studies, their credibility and ethics are under serious scrutiny.
C) The actual sample is not representative of the population. It is representative only of the elements of the population that chose to participate.
D) If many persons refuse to participate, they might refuse to participate in interventions that the study recommends.