题目内容：

Directions: Read the following passage. Fill in each blank with a proper sentence given in the box. Each sentence can be used only once. Note that there are two more sentences than you need.

Most of us would like to have a bright, “white” smile—filled with teeth that would make a Hollywood actor proud. Studies show that up to 52 percent of people are dissatisfied with the color of their teeth. 1. Globally, rows of pearl-white teeth shine out from magazines and cinema screens.

It is hardly surprising, then, that we tend to think that white teeth are not only attractive but also healthy. However, new studies have revealed that pearly teeth don’t always mean great oral health.

2. It can be based on our genes and ages. As you get older, your teeth often become yellower as the enamel（牙釉质）—the hard white outer layer of teeth—begins to wear away. Eating, drinking, smoking and taking certain medications might also discolor your teeth. These stains don’t actually indicate a problem with dental health. You can have pearl-white teeth and still have cavities（龋齿的洞）. Likewise, you can have perfectly healthy teeth that are yellowish or even brownish. 3.

There is even one type of stain that some researchers believe might protect against tooth decay. This is the “black stain” which you sometimes see along teeth at the edges of the gum line（牙龈线）. It can look like a series of dark dots. While the cause of the black stain has been in debate for more than a century, some studies have found that children with the stain are less likely to have tooth decay. It’s thought that some microbes in the stain might be somehow protective.

So what if you still want pearl-white teeth, regardless of what they say about your health? There is a wide range of products you can use. Some kinds of toothpaste promise a whitening effect. Most of them contain substances that rub stains off your teeth. Others add some blueish substances to your yellowish teeth, resulting in a bright white color. 4.

Of course, unless you continue using the product or receiving the whitening treatment, the whiteness will only last for as long as you avoid the foods and drinks that cause the stains in the first place. But now you know that even without that Hollywood white smile, it doesn’t mean you don’t have healthy teeth.

A.Many of the experiments on tooth color have been conducted in laboratory test tubes, rather than in the mouths of living people.

B.In countries such as the United States, tooth whitening is one of the most commonly requested dental treatments.

C.The color of our teeth is influenced by a number of factors.

D.It is no surprise that the red wine leaves the strongest stains.

AB. There are also professional whitening treatments, but whitening can cause temporary sensitivity in some people’s teeth or gums.

AC. Of course, in some cases, discoloration can indicate decay or other diseases, so it is worth getting stains checked out by a dentist.