What do you do if you are hit in the mouth and one of your teeth is knocked out? A dentist(牙医)may be able to put the tooth back in for you, but you have to get to the dentist first．In the meantime, you have to store the tooth so that it will stay healthy．
The simplest thing to do is put the tooth back into your mouth, or you could put it in milk．Frank Courts, a dentist at the University of Florida, worked with other researchers to find the safest way of keeping knocked-out teeth．He tried putting teeth into milk, into water, and into cell-culture medium(细胞培养液)．He also tried, just drying the teeth in the open air．He used teeth that had to be removed from people’s mouths for various reasons．The teeth were put into one of the solutions(溶液)or stored in the air for one hour．Then cells from the root of the teeth were scraped-off the teeth．Dr．Courts and the others then studied how healthy these cells were．
The cell-culture medium worked best to keep a knocked-out tooth healthy outside the mouth．However, few people keep cell-culture medium handy at home or in school．
The next best thing was milk．But Dr．Courts said that the best thing of all was to wash the tooth and put it right back into its socket(牙槽)in the mouth．For some people, though, this is just too painful or upsetting．So milk will do, until you get your tooth (and your mouth)to a dentist．
This passage is mainly about             ．

A．how a dentist can save a knocked-out tooth

B．how to keep a knocked-out tooth healthy

C．how Dr．Courts and other researchers made their experiments

D．how to put a knocked-out tooth back into its socket

The best way to keep a knocked-out tooth healthy outside the mouth is to store it       .

A．in cell-culture medium

B．in milk

C．in water

D．in the open air

Milk can be used instead of cell-culture medium to put a knocked-out tooth in because    ．

A．milk works better to keep the tooth healthy	B．milk causes less pain
C．cell-culture medium is not usually handy	D．cell-culture medium is more expensive

“Dr．Courts said that the best of al1 was to wash the tooth and put it right back into its socket in the mouth．”In the sentence, the word “best” means              .

A．cheapest and healthiest	B．least painful and cheapest
C．quickest and least painful	D．easiest and healthiest

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