题目内容：
根据材料，回答问题。Dogs are often 21 man's best friend. For those who are blind, a dog is 22 than a friendly company. A dog has a pair of sharp eyes. A 23 dog guide gives blind people the 24 to move through the world safely and freely. A dog guide is also a best friend. 25 a working team of a dog and a person is not easy. There are no factories making thousands of dogs guides a day. There are however, schools all 26 the country that train both blind people and the dogs they depend upon. The process of education is long for both the dog and its human partner.
A dog begins schooling as puppy. Around age of 8 weeks, the puppy is placed with a faster family. There it 27 training and gets used to 28 around people. When a dog is about a year old, it goes off to school for a series of tests. These tests help the school 29 whether the dog is likely to make a good guide. Dogs that fail the test are placed in permanent homes. 30 that passthe tests go into a tratnmg 31 .
For the next four-to-six months, the dog learns everything 32 needs to become a dog 33 . The dog learns to stay focused. It learns to ignore food, smell, and other animals. It learns to remain34 by such noises such 35 shouting or music from a passing car radio. It learns to 36 commands like"right", "left" or "forward". The dog learns to stop before every curb and stairway and to 37 moving objects. In short, good dog guides learn to keep their 38 safe while they go about the everyday actions of life. A dog that 39 completed training will be 40 a master.
A．call
B．calling
C．called
D．to call
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