In the early 20th century, a house named Clever Hans was believed capable of counting and other impressive mental task. After ye

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问题     In the early 20th century, a house named Clever Hans was believed capable of counting and other impressive mental task. After years of great performance, psychologists discovered that though Hans was certainly clever, he was not clever in the way 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 drug 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 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. Whiles 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 distracted 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、their dogs were expected to find the scents of red paper
D、some target scents may be labelled with a special mark

答案D

解析 由第二段中“And also that in two cases those scents would be marked by pieces of red paper.”可知,一些目标气味上可能有些特殊标记。故答案选D。
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