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Wisdom teeth are normally the last teeth to appear. This usually happens when people are in their late teen years or early twenties-in other words, when they are older and wiser.
Wisdom teeth are chewing teeth, at the back of the mouth. They can grow into place normally and never cause a problem. But often there is not enough room for them in the mouth. They might crowd other teeth. Sometimes they even push sideways through the gums(牙床).
An impacted wisdom tooth is one that fails to completely rise through the gum-the term is erupt. Wisdom teeth that only partially erupt can leave space for bacteria to enter around the tooth. Infection is a risk in these cases.
Wisdom teeth’that are not well in place and become impacted are often removed. People should have the placement of their wisdom teeth examined between the ages of sixteen and twenty. X-rays can show wisdom teeth that are below the gums.
The American Dental Association says remov
A. our ancestors needed to chew more when they grew up
B. our ancestors’ teeth grew at late teens or early twenties
C. our ancestors ate foods without cooking
D. our ancestors had very tough lives

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