Mouth bacteria grow fast in airless conditions. Oxygen rich saliva  keeps their numbers down. When we sleep, for example, the saliva stream slows, and sulfur  producing bacteria gains the upper hand, producing classic "morning breath".    Alcohol drinking, too much talking, breathing through the mouth during exercise, anything that dries the mouth produces bad breath. So can stress, though it's not understood why. Some people's breath turns sour every time they go on a job interview.    Saliva flow gradually slows with age, which explains why the elderly have more bad breath trouble than younger people do. Babies, however, who make plenty of saliva and whose mouths contain  ly few bacteria have characteristically sweet breath.    For most of us, the simple, dry mouth variety of bad breath is easily cured. Eating or drinking starts saliva and sweeps away many of the bacteria. Breakfast often stops morning breath.    Those with constant dry mouth find that it helps to keep gum, hard candy, or a bottle of water or juice around. Brushing the teeth wipes out dry mouth bad breath because it clears away many of the offending bacteria. Surprisingly, one thing that rarely works is mouthwash. The liquid can mask bad breath smell with its own smell, but the effect lasts no more than an hour. Some mouthwashes claim to kill the bacteria responsible for bad breath. The trouble is, they don't necessarily reach all offending germs. Most bacteria are well protected from mouthwash under thick layers of mucus  . If the mouthwash contains alcohol--as most do--it can worsen the problem by drying out the mouth. According to the passage, alcohol has something to do with bad breath mainly because   _  .
it keeps offending bacteria from reproducing its smell adds to bad breath it kills some helpful bacteria it affects the normal flow of saliva