It is bad to have food stuck between your teeth for long periods of time. This is because food attracts germs(细菌), germs produce acid, and acid hurts your teeth and gums. Flossing(使用牙线) helps to remove the food that gets stuck between your teeth. This explains why flossing helps to keep your mouth healthy, but some doctors say that flossing can be also good for your heart.
It may seem strange that something your do for your teeth can have any effect on your heart. Doctors have come up with a few ideas about how flossing works to keep your heart healthy. One idea is that the germs that hurt your teeth can leave the mouth and travel into your blood. Germs that get into the blood can then attack your heart. Another idea is based on the fact that when there are too many germs in your mouth, the body tries to fight against these germs. For some reason, the way the body fight these mouth germs may end up weakening the heart over time.
Not every doctor agrees about these ideas. Some doctors think that the link between good flossing habits and good heart health is only a coincidence. The incidence(发生率) of two or more events is completely random, as they do not admit of any reliable cause and effect relationship between them. For example, every time I wash my car, it rains. This does not mean that when I wash my car, I somehow change the weather. This is only a coincidence. Similarly, some doctors think that people who have bad flossing habits just happen to also have heart problems, and people who have good flossing habits just happen to have healthy hearts.
The theory that flossing your teeth helps to keep your heart healthy might not be true. But every doctor agrees that flossing is a great way to keep your teeth healthy. So even if flossing does not help your heart, it is sure to help your teeth. This is enough of a reason for everyone to floss their teeth every day.
From the passage, we know that_____

A．food stuck between your teeth may hurt your teeth

B．all of the doctors agree that flossing is good for your heart

C．doctors’ judgment about flossing is based on medical research

D．not every doctor thinks flossing helps to keep your teeth healthy

The underlined sentence in Paragraph 3 is to____.

A．Provide an example

B．Make a comparison

C．Introduce a new topic

D．Develop a previous statement

Which of the following best states the main idea of the last paragraph?_______.

A．It is a fact that flossing can help your heart as well as your teeth.

B．There is no good reason to believe that flossing will help your heart.

C．Because doctors find flossing won’t help your heart, it is useless to floss.

D．Even if flossing is only good for your teeth, you should still do it every day.

Which is the best title for the passage?

A．Flossing by Coincidence

B．How to Keep Your Teeth Healthy

C．Flossing Habits and Healthy Hearts

D．Why Doctors Disagree about Flossing