You are checking for underage drinking in a bar. There are four people at a table. You know that person A is drinking a coca-cola, person B is 16 years old, person C is drinking a cocktail, and person D is 25 years old.

Given that the drinking age is 21 (and minors are allowed in the bar), whose IDs must you check to ensure the law is not being broken? (Please enter the persons among the choices, A, B, C, D whose IDs must be checked.)