填空题Questions 20-24 Complete each sentence with the correct ending, A-J, below. Write the correct letter, A-J, in boxes 20-24 on your answer sheet. A．is the way most people around the world get fluoride. B．is a threat to public safety. C．can occur as a result of mishaps in adding fluoride to water. D．can be regarded as nutritious. E．is a widespread health issue in developed countries. F．is not as common in infants. G．is the best protection against tooth decay. H．is required to sustain our material survival and biological growth. I．is visible as faint lines across the teeth. J．is difficult to achieve amongst poorer people.
填空题
Tooth decay
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A constant, low level of fluoride in the mouth