Suppose You Are in Charge of Enforcing Safe Drinking Rules

Suppose you are in charge of enforcing safe drinking rules in a restaurant, which caters to both young and old. Obviously, the restaurant must abide by the law, which says: If you are drinking alcohol, then you must be 21 or above. Obviously, you cannot go around and check every single patron; so, you are constrained to a few random checks. What should you check for?

A) Check a few people drinking to confirm that they are 21 or above; check a few of those who look less than 21 to confirm that they are not drinking.
B) Check a few people not drinking to confirm that they are 21 or above; check those who look less than 21 to confirm that they are drinking.
C) Check a few people drinking to confirm that they are 21 or above; check a few people not drinking to confirm that they are below 21.
D) Check a few people above 21 to make sure that they are drinking; check a few people who look less than 21 to make sure they are not drinking.