question: What sentence would provide a factual answer to this question: "How low must be a population of some species drop before it is extinct?"

answer: if a population decreases to zero then that organism is extinct

question: What sentence would provide a factual answer to this question: "which of these people would be most susceptible to disease?"

answer: white blood cell destroys pathogens in the body

question: What sentence would provide a factual answer to this question: "Why does a turkey thermometer plunger cap come off?"

answer:
if gas is heated then that gas will rise