September 20was the 15th National Tooth Protection Day．On that day，more than 10，000young students gathered （聚集）in Shenzhen to clean teeth on the streets．
"It's exciting to brush teeth on the streets．But with so many people，I am a bit shy，w said Zhang Xi，a Grade 2student at Shenzhen Wenjin Middle School．
More importantly，she learned how to look after teeth．"I didn't know how io take care of my teeth before．Now，I will clean them longer and more carefully．"
Most teenagers clean their teeth every day，but there are still problems．It's because they often finish cleaning them in just one minute．A new Ministry of Health report shows that about half of the teenagers in China have problems such as decayed teeth （龋齿）．Some teenagers have bad eating habits and others do not know how to clean their teeth．
Li Xiaoyuan，a dentist，says there are many ways to protect teeth and he gives some good advice：
．Clean your teeth for three minutes each morning and evening and clean every side of your teeth carefully．
．Do not have too much sugar or soft drinks．
．Go to the dentist every six months for a check-up．
79．The students brushed teeth on the streets，because___．
A．they just played a game
B．they wanted to tell people how to protect the teeth
C．their teeth were dirty
D．their teachers made them do that
80．How did Zhang Xi feel on that day？___
A．Neither excited nor shy．
B．Either excited or shy．
C．Not only excited but also shy．
D．Not excited but shy．
81．You'd better brush your teeth for___every time．
A．one minute
B．two minutes
C．three minutes
D．four minutes
82．is good for your teeth．___
A．Eating lots of sugar
B．Drinking too much cola
C．Brushing your teeth as quickly as possible
D．Seeing a dentist every six months
83．Why do abo