At a buffet, a guest walks back to the line carrying the same plate they already ate from. What should the trained staff member do?

解析

Used plates carry saliva and food residue that can contaminate serving utensils and displayed food, and neither careful utensil handling nor a wipe with a cloth removes that risk — a plate that looks clean has still been eaten from. Self-service operations must require a clean plate for each return trip; beverage cups may generally be refilled if refilling avoids contact between the dispenser and the cup rim.