A、Forty to fifty minutes of exercise, once a week. B、Twenty to thirty minutes of exercise every day. C、Fifteen to twenty minutes

Diet, diet, diet! It gets to be a constant battle for many of us to stay fit and healthy. But diets don’t always allow us to learn new eating habits, improve our self-image, or make lifestyle changes. Developing good eating habits and proper exercise is a permanent issue. And it’s the main topic of this workshop.
    In order to keep a healthy diet and minimise the risk of disease, it’s essential to eat a diet that consists of low-fat foods and to maximise energy with carbohydrates. Beef, nuts, oils, and most dairy products are rich in fat, so you should eat less of those; while pastry, rice, potatoes, and bread contain almost no fat and are a high-energy source of complex carbohydrates. You know that many food manufactures use sugar to make low-fat or fat-free foods taste good, but don’t worry too much about that. You don’t necessarily "get fat" by eating sugar, since sugar is an indirect fat.
    When you exercise, carbohydrates are the first calories to burn. If you want to burn off fat, it’s good to do some sort of aerobic exercise such as biking, swimming, running, or fast walking. But you have to keep doing this exercise for at least twenty minutes because it takes about that long before we start to burn fat. And then the effect of burning fat decreases after 45 minutes of exercise. So if you do some sort of aerobic exercise for three to four days a week for about thirty to forty minutes, you’ll have an effective schedule for reducing weight.
Questions 20 to 22 are based on the passage you have just heard.
20. According to the speaker, which should you eat more?
21. What does the speaker say is burned at the beginning of exercise?
22. According to the speaker, what is the most effective exercise time schedule for reducing weight?