question: What sentence would provide a factual answer to this question: "Salt was at one time"

answer: adding salt to a solid decreases the freezing point of that solid

question: What sentence would provide a factual answer to this question: "In a warm room, it is likely that the source of heat is"

answer: a radiator is a source of heat

question: What sentence would provide a factual answer to this question: "Where would a greenhouse best be used?"

answer:
a greenhouse is used to protect plants by keeping them warm