D．weak

　　解析　与the strong对应，故选D。

　　答案　D

　　A

　　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 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 (齿龈)．

　　An impacted wisdom tooth is one that fails to completely rise through the gums.Wisdom teeth that only partly break through can leave space for bacteria to enter around the teeth.Infection (感染) is a risk in these cases.

　　Wisdom teeth that are not well lined and become impacted are often removed.The American Dental Association says removal is generally advised when wisdom teeth only partly break through the gums.Removal is also advised if there is a chance that wisdom teeth ate poorly lined.The best time to remove is before the teeth cause any problems or pain.Young adults are the best candidates for wisdom teeth removal.

　　But why do we have wisdom teeth if we often need to get them removed？One theory has to do with our diets.Scientists say the diet of ancient humans probably required more chewing teeth.Life was probably a little rougher on the teeth back then，too.So it was good to have extras.

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