有A、B、C三条铜导线A、B长度相等，B比A粗，A、C粗细相同，C比A长，则三条

Directions: Read the following text and choose the most suitable heading from A-F for each paragraph．There is one extra heading which you do not need.

A．Causes of overusing fluoride	B．Negative effects of fluoride
C．Safe use of fluoride	D．The expert’s background information
E．No need to worry about fluoride poisoning
F．Solutions to improper use of fluoride

I have heard that fluoride can help stop dental decay, but I recently read that it is poisonous．Which of these is true? How can children use fluoride safely?

小题1:

This question is answered by Dr Marcia M．Rich who practises general dentistry in Newholt．She is also a lecturer at the University and a writer for the monthly magazine Your Dentist Cares．

小题2:

My answer to the first question is to stress that fluoride works well and is safe when users follow instructions．Young children have a tendency to swallow toothpaste, which is why they should only have a tiny amount of fluoride toothpaste on their toothbrush．If this simple measure is taken, parents can be confident that children will be protected against tooth decay, and come to no harm.

小题3:

Almost all medicines have an adverse effect－or even be deadly－if they are not taken as directed．So you are right－fluoride can be poisonous if it is swallowed in very large quantities．It is for the reason that fluoride supplements can only be obtained on prescription from the doctor or dentist．Please be assured that fluoride overdoses are rare－in fact, I have never heard of any fatal incidents related to fluoride poisoning.

小题4:

Scientific studies have shown that fluoride can help to prevent cavities as long as it is used correctly．If fluoride is abused, there is a risk of illness or even deaths in extreme cases．If a deadly amount of fluoride is taken, immediate first-aid could save a life．A person who has swallowed a large amount of fluoride will probably start vomiting．If not, the patient should be given milk or antacid and taken to the nearest hospital for emergency treatment.

小题5:

Taking too much fluoride while teeth are growing can lead to a condition called dental fluorosis．The most likely cause is the swallowing of fluoride toothpastes by young children．It can also be caused by inappropriate use of fluoride drop or other fluoride supplements, for example when fluoride is already is already present in drinking water．