In this task, you are given a question and an answer, you would be asked to create the sentence based on the Question-Answer provided. It should be contained within the Question-Answer provided.

[EX Q]: Question: What occurs when a substance reacts quickly with oxygen? Answer: combustion reaction
[EX A]: Combustion reaction occurs when a substance reacts quickly with oxygen.

[EX Q]: Question: Which has a higher metabolic rate, a mouse or an elephant? Answer: mouse
[EX A]: Mouse has a higher metabolic rate, a mouse or an elephant.

[EX Q]: Question: When food enters the stomach after a period of fasting, a drop in what initiates the conversion of pepsinogen to pepsin? Answer: ph
[EX A]:
Ph food enters the stomach after a period of fasting, a drop in what initiates the conversion of pepsinogen to pepsin.