A hostel warden argues that serving the evening meal half an hour later will cut the amount of food thrown away, because students who now walk into the mess straight from the playing field are too tired to eat and leave their plates full. Which of the following is an assumption on which the warden's argument depends?

The whole benefit rests on the delay doing its work: if half an hour changes nobody's appetite, the plates come back just as full and nothing is saved. Say that the assumption is false and the argument collapses at once. Who cooks the food does not matter; the third option is far stronger than the argument needs, since a few snackers would not defeat it; and what students would prefer is not what the argument claims.