[originaltext] Cattle have served humanity since prehistoric days as beasts

Cattle have served humanity since prehistoric days as beasts of burden and as suppliers of leather, meat, and milk. Some of the earliest written records concern the sale of cattle. These valuable animals are unusual in that they do not have front teeth in their upper jaw. Instead they chew with their rear teeth and gums. Cows swallow their food quickly and store it in the paunch or rumen, the first of the four compartments in their stomachs. Later the food passes into the second stomach or reticulum where it is rolled into little balls or cuds. While resting, cows cough up these cuds and chew them more thoroughly. This time the food passes into the third and then into the fourth stomach, where digestion takes place. Bacteria in cow’ s stomachs aid in digesting the cellulose in stems of grass or hay.
    Over one hundred million head of cattle are raised in the United States. Dairy cattle produce more than fifteen and one-half billion gallons of milk every year. Although dairy cattle are bred primarily to produce milk, about half the beef used in the United States comes from dairy breeds. This is because when dry(no longer producing milk)they fatten quickly and produce high-quality beef.
Questions 16 to 18 are based on the passage you have just heard.
16. According to the passage, what is the rumen?
17. What is the cow’ s second stomach used primarily for?
18. Which of the following is essential in the digestion of cellulose in the cow’ s stomach?