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 from 39 to 43.
Some doctors think that you should drink a glass of water each morning. You should drink this water first thing, before doing anything else. The temperature of the water should be similar to body temperature; neither too hot nor too cold.
Why should you drink this water? Water helps your body in many ways. It helps clean out your kidneys. It prepares your stomach for digestion. Water can also help your intestines work better. After drinking water, the intestines can more easily take out nutrients from our food. Water also helps us go to the bathroom more easily.
Scientists suggest that people take in 1,600 milliliters of water each day. But don't drink all of that water in one sitting. If you do, your kidneys will have to work much harder to eliminate it. It's better to drink some in the morning and some in the afternoon. Some people think it's better to drink between meals and not during meals. They think water dilutes the juices produced in our stomachs. This can interfere with normal digestion.
Are you drinking enough water every day? Check the color of your urine. If it is light yellow, you are probably drinking enough. If your urine is very dark yellow, you probably need to drink more water. A little more water each day could make you much healthier.
What is the main idea of the passage?
A. The importance of water
B. The advice of the doctors
C. The best amount of water to drink
D. How to drink water correctly