In the early 20th century, a horse named Clever Hans was believed capable of counting and other impressive mental tasks. After y

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问题     In the early 20th century, a horse named Clever Hans was believed capable of counting and other impressive mental tasks. After years of great performance, psychologists discovered that though Hans was certainly clever, he was not clever in the way that everyone expected. The horse was cleverly picking up on tiny, unintentional bodily and facial signals given out not only by his trainer, but also by the audience. Aware of the "Clever Hans" effect, Lisa Lit at the University of California and her colleagues wondered whether the beliefs of professional dog handlers might similarly affect the outcomes of searches for drugs and explosives. Remarkably, Dr. Lit found, they do.
    Dr. Lit asked 18 professional dog handlers and their dogs to complete brief searches. Before the searches, the handlers were informed that some of the search areas might contain up to three target scents, and also that in two cases those scents would be marked by pieces of red paper. What the handlers were not told was that none of the search areas contained the scents of either drugs or explosives. Any "detections" made by the teams thus had to be false.
    The findings reveal that of 144 searches, only 21 were clean (no alerts). All the others raised one alert or more. In total, the teams raised 225 alerts. While the sheer number of false alerts struck Dr. Lit as fascinating, it was where they took place that was of greatest interest.
    When handlers could see a red piece of paper, allegedly marking a location of interest, they were much more likely to say that their dogs signalled an alert. The human handlers were not only distracted on almost every occasion by the stimulus aimed at them, but also transmitted that distraction to their animals—who responded accordingly. To mix metaphors, the dogs were crying "wolf at the unconscious signal of their handlers.
    How much that matters in the real world is unclear. But it might. If a handler, for example, unconsciously "profiled" people being sniffed by a drug—or explosive-detecting dog at an airport, false positives could abound. That is not only bad for innocent travellers, but might distract the team from catching the guilty.
The dog handlers learned before the searches that

选项 A、each search area contained three target scents.
B、there was actually no target scent in the search area.
C、some target scents may be labelled with a special mark.
D、their dogs were expected to find the scents of red paper.

答案C

解析 细节题。根据文章第二段第二句“Before the searches,the handlers…marked by pieces of red papers.”可知,搜查之前,训犬员被告知一些搜查区域可能会有多达三处的目标气味.而且有两处会用红纸作为记号。所以正确答案为C项。
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