(D)

    Losing a baby tooth can be fun for a kid. You can put it under your pillow and look forward to finding some money there in the morning. And then you can watch your new permanent(永恒的) tooth grow in its place.

    But if a permanent tooth falls out, that's a problem. There isn't another one under the gum(齿龈)that can replace it. Unluckily, this happens to some older people. And if permanent teeth fall out or need to be pulled, an older person might need dentures.

    Dentures, known as false(假的) teeth, are a set of replacement teeth for any teeth that are missing. There are partial dentures, which take the place of only a few teeth and prevent the others from changing position. If all the teeth are gone, the person would need complete dentures, which replace every tooth in the mouth.

    Dentures can cause pains at first and take little time to get adjusted to. They also need special care. They should be brushed daily with a special denture brush and cleanser and soaked(浸泡) in denture solution(溶液) when not in the mouth.

    Dentures are important for the old because, without teeth, it's hard to smile, talk, and eat properly. But it can be a little weird if you see someone take out his or her dentures. It can also be strange to see someone, like a grandparent, without dentures in because he or she won't have any teeth. Try not to make fun of the person because this could cause hurt feelings.

    People usually lose their permanent teeth due to periodontal disease（牙周炎）, or gum disease, which is caused by bacteria that attacks the gums. Diseased gums aren't strong and healthy, so teeth can get loose and fall out. Older people often have many cavities(洞)that have been filled over the years, and these fillings can weaken over time, leading to more tooth problems.