阅读下面 A 、 B 、 C 、 D 四篇短文 , 掌握其大意 , 然后从所给的四个选项中选出最佳选项。

[ A ]

A dog is a useful animal for human beings. Training a dog to be a watchdog often produces unexpected effects.

Some dogs quickly learn the 1 between unwanted people and friends. This is 2 their masters welcome their friends and invite them to their houses. 3 , some dogs will always jump at the postman who comes to deliver letters. Why? One explanation for this behavior is that 4 the postman comes to the house often, he never enters the house. Because of this, the dog thinks the postman is someone 5 is not wanted, but keeps coming back anyway.

Masters of the dogs that attack the postman can 6 show the dog that the postman is a friend and that the dog doesn't need 7 him as an unwanted person. A dog is quite ready to do 8 his master wishes. And a dog is always happy when he is praised for understanding correctly.

Dogs are also useful as companions( 伙伴 ) for blind people. When a dog has 9 properly trained, he will always lead his blind master in the right direction and keep him out of 10 . For example, seeing-eye dogs learn never to cross a busy road when cars are coming, even if their masters order them to do so.

1. A. different B. difference C. secret D. results
2. A. because B. why C. how D. when
3. A. Or B. But C. However D. Therefore
4. A. if B. since C. as D. although
5. A. who B. what C. which D. whether
6. A. hard B. difficult C. easily D. hardly
7. A. think of B. regarding C. considered D. to treat
8. A. what B. that C. which D. who
9. A. be B. to be C. being D. been
10. A. safe B. safety C. danger D. dangerous