What is the main idea of the passage?

Câu hỏi :

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. 

     Apples are one of the healthiest foods you can eat every day. A traditional American proverb states: "An apple a day keeps the doctor away". But it does not specifically say anything about the best time of day to eat apples. It is generally believed that eating an apple in the morning is good for people, but eating an apple in the evening is like eating poison. 

     There is actually a scientific reason behind this belief. One benefit of apples is that they are a favorable source of fructose, which is a natural form of sugar that gives you a big boost of energy. The energy you get from an apple is longer lasting than that provided by a cup of coffee. Clearly, this is something most people would rather experience in the morning than right before bedtime. 

     Apples also contain high levels of pectin, a kind of dietary fiber. Fiber stimulates bowel movements, which help maintain the health of your digestive system. But if you eat an apple shortly before going to bed, this can cause problems. While you are asleep, your bowels can fill up with gas, making you feel bloated and uncomfortable. You may also wake up several times to use the bathroom, which will prevent you from getting a good night's sleep. 

     Other health benefits of apples come from flavonoids, beta carotene, and B vitamins. Flavonoids are the material that gives flowers and fruits their bright colors, such as the deep red of apples. When eaten, flavonoids can decrease your risk of heart disease, diabetes, and other illnesses. Meanwhile, beta carotene helps prevent cancer, and B vitamins provide the body with a wide variety of important benefits. Apparently, starting your day with an apple really can keep the doctor away. 

What is the main idea of the passage?