A、When they are hungry. B、When they are thirsty. C、When they smell food. D、When they want company. A浏览四个选项，从选项中表示时间的连词when以及hung

Have you ever felt like you would do just about anything to satisfy your hunger? (16) A new study in mice may help to explain why hunger can feel like such a powerful motivating force. In the study, researchers found that hunger outweighed other physical drives, including fear，thirst and social needs.
To determine which feeling won out, the researchers did a series of experiments. In one experiment, the mice were both hungry and thirsty. When given the choice of either eating food or drinking water, the mice went for the food, the researchers found. However, when the mice were well-fed but thirsty, they opted to drink, according to the study. In the second experiment meant to pit the mice’s hunger against their fear. Hungry mice were placed in a cage that had certain fox-scented areas and other places that smelled safer (in other words, not like an animal that could eat them) but also had food. (17) It turned out that when the mice were hungry, they ventured into the unsafe areas for food. But when the mice were well-fed they stayed in areas of the cage that were considered safe. Hunger also outweighed the mice’s social needs, the researchers found. (18) Mice are usually social animals and prefer to be in the company of other mice, according to the study. When the mice were hungry, they opted to leave the company of other mice to go get food.
Questions 16 to 18 are based on the passage you have just heard.
16. What is the researchers’ purpose in carrying out the series of experiments with mice？
17. In what circumstances do mice venture into unsafe areas？
18. What is said about mice at the end of the passage？