Is anyone out there? Can anyone save me from this math problem misery/ I think its pre algebra

A hospital mails out surveys to 500 recent patients to get feedback on their hospital visit.

A. population-recent patients at the hospital
sample-people who mail back survey
possible bias-higher hospital bills for some may skew their response

B. population-recent patients at the hospital
sample-people who mail back survey
possible bias-extremes more likely to respond

C. population-community around the hospital
sample-recent patients
possible bias-only recent patients can respond

D. population-people who mail back survey
sample-random pick from the responses
possible bias-extremes more likely to respond

it's A.
That's the only one (possible bias) that made any sense....
gl

I think the answer would be C.