What is the meaning of harmful substances in food production

Well it means that if you eat something bad you can get sick and might get different problems then just that but some examples of harm substances are: stale food or food that is spoiled. expired food and drugs/ medicine. impure water which is water that hasn't been ran threw a filter or nasty water. unripe food that you aren't spouse to eat yet. foods that are infested with bugs or been eaten by another animal. food that isn't folly cooked and cigarettes. 

Your answer is  d.)relationship between pressure and air volume and led to the understanding of how respiration works