题目

Guide dogs (导盲犬) are a big help to blind people. A guide dog helps its owner cross the street and do many other things. But it is not easy to become a guide dog. Here’s what a dog must do to get the job: 1. When the dog is very young, trainers watch the dog to make sure it is friendly and quiet. 2. For one year, the dog goes to live with a testing (试验) family. The family teaches the dog some simple words, like “Sit!” and “Stay!” 3. Then the dog goes to school, where trainers teach the dog to lead a person. The dog must learn to stay clear of cars and other dangers in （1）石墨与金刚石性质不同是由于它们的    不同．（2）生活中可以通过    方法来降低水的硬度．（3）建筑工人通常在用剩的石灰浆上面覆盖一层沙土的原因是    ．