根据下面资料，回答题
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 cher-ished 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 carded out(26)____ ravens.
(27)____ to gaze is reckoned to be a good(28)____ of the development of theory of mind in hu-man children. (29)____ about 18 months, most children are able to follow the gaze of another per-son, 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.
第21题选