Passage 4 Questions 16 to 20 are based on the following passage: Animals seem to have the sense to eat when they are hungry and they do not eat more than their bodies need. It has been demonstrated that rats will, when given a choice over a period of time, prefer water with vitamins to water without vitamins even though there is no difference in taste or smell between the two water bottles. When a fragrant flavor was added to the vitamin-enriched fluid, the rats did seem to develop a taste for it and kept drinking it, even after the vitamins were switched to the clear water. In time, however, they broke the habit and went back to where the necessary vitamins were. In a classic experiment, babies of 6 to 12 months old were placed in a cafeteria feeding arrangement, with a wide selection of baby food before them. They were given whatever food they pointed to or appeared interested in. We are told that at first they showed some unusual eating patterns, but that over a period of time they managed to select well-balanced diet. So, in selecting food, rats and babies do seem to know and act on what’s best for them. Apparently, there is a kind of “body wisdom,” which humans soon lose. Most of us do not eat as wisely as we could. Many of our food preferences are culturally determined and influenced by long-established habits. Some people eat fox, dog and blackbirds, while we eat cows and pigs. So what people eat and how much they eat seems to be greatly influenced by what is going on around them. 第16题：In the experiment on rats, a fragrant flavor was added to the rat’s drinking water to ________