根据以下内容，回答下列各题。 Persons who are overweight should watch their diet carefully i

n order to lose pounds. The best way to do this is to start a weight control program. At first it is wise to talk with your doctor. He can advise you of the number of calories(卡路里) you should have in your meals each day. He can tell you about exercisingwhile on your diet. A good rule is to lose slowly. A loss of a pound or two is plenty. Plan meals around foods you know. This means that it is wise to include foods that you are used to and that are part of your regular eating habits. When you have lost the weight you wish, simple items can be added to your diet so that you can maintain the weight you want. While you are dieting, try to build a pattern of eating that you can follow later to maintain your desired weight. When dieting, choose low-calorie foods. Avoid such items as fats. fried food, sweets, cakes, cream and soft drinks. Try to take coffee and tea without sugar or cream. Snacks can be part of your diet. For example, a piece of fruit or a simple dessert saved from mealtime can be eaten between meals. Keep busy ! This way you will not be tempted to go off the diet. Make full use of opportunities to exercise. Try walking instead of riding whenever possible. Happy dieting! What is the main idea of the passage？

A.How to diet.

B.Why we should diet.

C.What we should diet.

D.Where we should diet.