单项选择题

The murder took place around ten p. m. on June 10. Thirty-two people watched Kitty being killed right beneath their windows. She was their neighbour. Yet, none of the 32 helped her. Not even one called the police. Was this inhuman cruelty (狠心) Was it lack of feeling for one’s fellow man
"Not so," say scientists Dr. Darley and Dr. Fatane. They’ve found the reasons why people don’t act. According to them, a person has to go through two steps before he can help. First he has to notice there is an emergency (紧急情况).
Suppose you see a middle-aged man fall onto the ground. Is he having a heart attack, or some other physical trouble Or is he simply drunk (醉酒) So it’s not always easy to tell if you are faced with a real emergency.
Second and more important, the person faced with an emergency must feel himself responsible. He must feel that he must help.
The researchers found that a lot depends on how many people are around. They asked college students to come in to be "tested". Some came alone, some with one or two others, and some in large groups. When they came in, either alone, in pairs, or in groups, a lady went into the next room. Soon the students heard a scream, the noise of something falling and a cry for help. All of this had been pre-recorded on a tape-recorder.
Eight out of ten of the students taking the test alone acted to help. Of the students in pairs, only two out of ten helped. Of the students in groups, none helped.
In other words, in a group, Americans often fail to act. They feel that others will act. They, themselves, needn’t. They do not feel any direct responsibility.

Americans don’t（）in a group.

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