A、Produce green food. B、Waste no food. C、Quit eating meats. D、Grow vegetables. C短文最后提到，很多人意识到为了改善环境、帮助人类生存，更多的人有必要成为素食者，即放弃吃肉，故C

(20) A recent study shows that meat consumption is one of the main ways that human can damage the environment, second only to the use of motor vehicles. So how can eating meat have a negative effect on the environment? For a start, all animals, such as cows, pigs and sheep, release gas named methane, which is the second most common greenhouse gas after carbon dioxide. Many environmental experts now believe that methane is more responsible for global warming than carbon dioxide. It is estimated that 25% of all methane released into the atmosphere comes from farm animals. Another way in which meat production affects the environment is through the use of water and land. 2,500 gallons of water are needed to produce one pound of beef, whereas 20 gallons of water are needed to produce one pound of wheat. One acre of farmland used for raising cows can produce 250 pounds of beef. One acre of farmland used for crop production can produce 50,000 pounds of tomatoes. Many people now see the benefits of switching to vegetarian diet which excludes meat and fish, not just for health reasons, but also because it plays a vital role in protecting the environment. (21) However, some nutritionists advise against switching to a totally strict vegetarian diet. They believe such a diet which includes no products from animal sources can be deficient in many of the necessary vitamins and minerals our bodies need. (22) Today many people have come to realize that to help the environment and for the human race to survive, more of us will need to become vegetarian.
20. What does the recent study show?
21. What do some nutritionists say about the strict vegetarian diet?
22. What does the speaker think more people need to do?