To settle their stomachs, young wild dogs often eat a particular type

To settle their stomachs, young wild dogs often eat a particular type of grass. This grass has properties that prevent certain types of illness-causing bacteria from proliferating in their intestines. When the dogs have reached maturity, though, they seldom eat grass, and thus a significant amount of the bacteria can be found in their intestines. Strangely enough, in areas where little grass is available for consumption, few young dogs are found to have the illness caused by the bacteria.
Which of the following, if true, most helps to explain the strange finding?

A. Grass does not eradicate populations of bacteria already present in the intestines of wild dogs.

B. Bacteria infect dogs most often in areas where there is little grass available for consumption.

C. The amount of bacteria present in the intestines of wild dogs varies wildly over the span of a year.

D. Bacteria, present in large enough quantities, can be lethal to infected dogs.

E. Young wild dogs in these areas expel the bacteria orally before it can cause illness.