A．trainedB．to trainC．trainingD．train

单项选择题

Dogs are often 1 man"s best friend. For those who are blind, a dog is 2 than a friendly company. A dog has a pair of sharp eyes. A 3 dog guide gives blind people the freedom to move through the world safely and freely. A dog guide is also a best friend. 4 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 5 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 6 training and gets used to being 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 7 whether the dog is likely to make a good guide. Dogs that fail the test are placed in permanent homes. 8 that pass the tests go into a training 9 .
For the next four-to-six months, the dog learns everything it needs to become a dog 10 . The dog learns to stay focused. It learns to ignore food, smell, and other animals. It learns to remain 11 by such noises such as shouting or music from a passing car radio. It learns to 12 commands like "right", "left" or "forward". The dog learns to stop before every curb and stairway and to 13 moving objects. In short, good dog guides learn to keep their 14 safe while they go about the everyday actions of life. A dog that 15 completed training will be assigned a master.

A．trained
B．to train
C．training
D．train