19．Are you happy with your teeth？Becca，a 15-year-old girl from the US is not．She decided her teeth needed to be whiter．"They weren't that bad，but I wanted to make them all the same colour"，she said．
In the last five years，teeth whitening has become very popular among young Americans．Now the trend （潮流） has come to China．If you turn on the TV you can see the Hong Kong pop star Karen Mok showing her bright white teeth，saying，"want to be like me？Use white teeth strips （牙贴）!"You may even have decided to buy one．
But wait a minute．Dentists have something to say．They think that kids should not use whitening products until they are 16or even older．
"We don't know what will happen yet，"says Dr．Nasrin Azim，a dentist in Chicago．"Studies have not been done on young people．I wouldn't do teeth whitening on anyone under 18．"
The problem with whitening lies in the pulp （牙髓），the innermost （最深处的） part of the tooth．It has the tooth's nerve （神经） ending．When we eat very hot or cold things，it's the pulp that hurts．Kids have larger pulps．The larger pulp might make teeth more sensitive （敏感的） to whitening and get hurt．
Another problem is that the gums around the teeth usually change shape （形状） with age．So，if someone whitens his or her teeth during childhood，the darker，natural colour might show later around the gum line．At that time，your teeth will show two colours．And the darker colour is difficult to whiten．
So，wait another few years before you make your teeth whiter．Or if you really want to do it，ask your dentist．They'll give you useful advice．
61．This passage mainly talks aboutD．
A．how to whiten your teeth
B．the new trend of whitening teeth
C．Americans with white teeth
D．dentists'advice on tooth whitening
62．The show done by Karen Mok is probably a（n）C．
A．notice
B．announcement
C．advertisement
D．talk show
63．When we eat hot or cold food，the pulp hurts becauseC．
A．it is inside the teeth
B．it is sick
C．it is rich in nerves
D．it is not used to this kind of food
64．What may happen if children do tooth whitening according to dentists？A
A．Their teeth may show two colours later．
B．Their gums may change shape with age．
C．Their teeth may get darker．
D．They may lose their teeth．
65．We can infer from the passage thatB．
A．dentists like very much the idea of tooth whitening
B．more and more people in China will get their teeth whitened
C．it's necessary for children to whiten their teeth
D．tooth whitening will certainly bring good results for people．
