Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.

The food we eat seems to have profound effects on our health. Although science has made enormous steps in making food more fit to eat, it has, at the same time, made many foods unfit to eat. Some research has shown that perhaps eighty percent of all human illnesses are related to diet and forty percent of cancer is related to the diet as well, especially cancer of the colon. People of different cultures are more prone to contact certain illnesses because of the characteristic foods they consume.

That food is related to illness is not a new discovery. In 1945, government researchers realized that nitrates nitrites (commonly used to preserve color in meat) as well as other food additives caused cancer. Yet, these carcinogenic additives remain in our food, and it becomes more difficult all the time to know which ingredients on the packaging label of processed food are helpful or harmful.

The additives that we eat are not all so direct. Farmers often give penicillin to cattle and poultry, and because of this, penicillin has been found in the milk of treated cows.

Sometimes similar drugs are administered to animals not for medical purposes, but for financial reasons. The farmers are simply trying to fatten the animals in order to obtain a higher price on the market. Although the Food and Drug Administration (FDA) has tried repeatedly to control these procedures, the practices continue.

A healthy diet is directly related to good health. Often we are unaware of detrimental substances we ingest. Sometimes well-meaning farmers or others who do not realize the consequences add these substances to food without our knowledge.

The word “these” refers to___________?