[单项选择]

We all have offensive breath at one time or another. In most cases, offensive breath emanates from bacteria in the mouth, although there are other, more surprising causes.
Until a few years ago, the most doctors could do was to counsel patients with bad breath about oral cleanliness. Now they are finding new ways to treat the usually curable condition.
Bad breath can happen whenever the normal flow of saliva (唾液) slows. Our mouths are full of bacteria feeding on protein in bits of food and shed tissue. The bacteria emit evil-smelling gases, the worst of which is hydrogen sulfide (硫化物).
Mouth bacteria thrive in airless conditions. Oxygen-rich saliva keeps their numbers down. When we sleep, for example, the saliva stream slows, and sulfur-producing bacteria gain the upper hand, producing classic" morning breath".
Alcohol, hunger, too much talking, breathing through the mouth during exercise -- anything that dries the mouth produces bad brea
A. Tooth trouble.
B. Sulfur-rich food.
C. Too much exercise.
D. Mental strain.

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