4、Most schools ban chewing gum，but in a few years they might consider changing that rule．Why？ Scientists are finding evidence that gum chewing may be good for your health．It may even help boost（提高）your test scores．

This exciting research is just beginning．And in the meantime，companies are also experimenting with adding vitamins，minerals，medicines，and other substances that could give gum the power to cure headaches and fight everything from serious diseases to bad breath．

People have been chewing gum for thousands of years．Ancient Greeks，Mayas，and Native Americans，for example，chewed on the sap，or resin of certain types of trees．Today gum is a litde more complicated．Gum manufacturers start by mixing resin，wax，and a molecule called polyvinyl acetate to make a gum base．By varying the types and amounts of these ingredients，scientists can make thousands of formulations．

Just what can gum do for your health？Many studies show that chewing gum after meals fights cavities（洞）by stimulating the production of saliva．Saliva helps wash away bacteria that damage our teeth．An ingredient called xylitol，which is added to it，provides an extra dose of cavity－fighting power．This is good for more than just the teeth．Research suggests that good oral health decreases the risk of heart disease，and other serious diseases．

Chewing gum might also be good for your brain．Chewing gum boosts the flow of blood to participants’brains by 40 percent．

But too much chewing can damage the jaw joint．And chewing too much of gum that contains vitamins，caffeine，or a medicine could lead to an overdose．What’s a more，no matter how healthy gum chewing proves to be，it will never be a match for a healthy lifestyle．

Nothing like gum or a vitamin is going to cure a bad dietary habit or a bad exercise habit；it’s not a magic bullet．

 

64．Why are companies trying to put medicines into gum？

A．Because they want gum to cure some diseases．

B．Because they wish people to become addictive to it．

C．Because they plan to make gum more tasteful．

D．Because they hope to make gum good for teeth．

65．From the last two paragraphs，we can infer that      ．

A．chewing gum has a magic power to cure your bad lifestyle

B．chewing gum can’t  replace the healthy lifestyle

C．chewing gum can lead to a bad exercise habit

D．too much gum chewing proves to be harmful to your teeth

66．How does chewing gum fight cavities？

A．By making people brush teeth immediately after meals．

B．By killing bacteria that are harmful to our teeth．

C．By making people produce more saliva．

D．By adding some more medicines to the gum．

67．The best title for the passage may be      ．

A．A Match for Exercise                                  B．Gum Chewing is a Magic Bullet

C．No Gum Chewing at School                        D．Chewing Gum for Health