A、When they are no longer useful. B、When they become ill. C、When they become escapees. D、When they get too old. C选项表示某种特征，根据前两题，

The University of Tennessee’s Walters Life Sciences building is a model animal facility, spotlessly clean, careful in obtaining prior approval for experiments from an animal care committee. Of the 15,000 mice house there in a typical year, most give their lives for humanity. These are good mice and as such won the protection of the animal care committee. At any given time however some mice escape and run free. These mice are pests. They can disrupt experiments with the bacteria organisms they carry. They are bad mice and must be captured and destroyed. Usually, this is accomplished by means of sticky traps, a kind of fly paper on which they become increasingly stuck. But the real point of the cautionary tale, says animal behaviorist Herzau, is that the labels we put on things can affect our moral responses to them. Using stick traps or the more deadly snap traps would be deemed unacceptable for good mice. Yet the killing of bad mice requires no prior approval. Once the research animal hits the floor and becomes an escapee, says Herza, its moral standard is instantly diminished. In Herzau’s own home, there was more ironic example when his young son’s pet mouse Willy died recently, it was accorded a tearful ceremonial burial in garden. Yet even as they mourned Willy, says Herzau, he and his wife were setting snap traps to kill the pest mice in their kitchen with the bare change in labels from pet to pest; the kitchen mice obtained totally different moral standards.
Questions 9 to 12 are based on the passage you have just heard.
9. What does the passage say about most of the mice used for experiments?
10. Why did the so-called bad mice have to be captured and destroyed?
11. When are mice killed without prior approval?
12. Why does the speaker say what the Herzau’s did at home is ironical?