题目内容：
根据材料，回答下列问题。Humans like to regard themselves as exceptional.Many philosophers believe humans are the only 21 which understands that others have their own personal thoughts.That understanding is known in the 22 as having a "theory of mind," and it is considered the 23 to such cherished human 24 as sympathy and deception.
Biologists have learned to treat such 25 with caution.Bernd Heinrich and Thomas Bugn-yar describe an experiment they have carried out 26 ravens.
27 to gaze is reckoned to be a good 28 of the development of theory of mind in human children. 29 about 18 months, most children are able to follow the gaze of another person, and 30 things about the gazer from it.Failure to 31 this trick is an early symptom of autism, a syndrome whose main underlying feature is a(n) 32 to understand that other people have 33 , too.
To 34 whether ravens could follow gaze, Dr Heinrich used six six-month-old hand-reared ravens.The birds were set, one at a time, on a perch on one side of a room divided by a barrier.An experimenter in front of the barrier 35 his head and eyes in a particular 36 and gazed for 30 seconds before looking 37 Dr Heinrich found that all the birds were able to follow the gaze of the experimenters, even 38 the barrier.In the 39 case, the curious birds either jumped down from the perch and walked around the barrier to have a 40 or leapt on top of it and peered over.
A．category
B．group
C．race
D．species
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