A、When they are no longer useful. B、When they become ill. C、When they become escapees. D、When they get too old. C短文中提到，消灭坏老鼠是不需要

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 housed there in a typical year, most give their lives for humanity. These are "good" mice and as such were 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 bacterial 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 flypaper, on which they become increasingly stuck. But the real point of this cautionary tale, says animal behaviorist Herzog, 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 a research animal hits the floor and becomes an escapee, says Herzog, its moral standing is instantly diminished.
    In Herzog’s own home, there was a more ironic example. When his young son’s pet mouse Willie died recently, it was accorded a tearful ceremonial burial in the garden. Yet even as they mourned Willie, says Herzog, 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 attained a totally different moral status.
Questions 26 to 29 are based on the passage you have just heard.
26. What does the passage say about most of the mice used for experiments?
27. Why did the so-called bad mice have to be captured and destroyed?
28. When are mice killed without prior approval?
29. Why does the speaker say what the Herzogs did at home is ironical?