单项选择题

Task 1
Directions: After reading the following passage, you will find 5 questions or unfinished statements, numbered 36 to 40. For each question or statement there are 4 choices marked A, B, C and D. You should make the correct choice and mark the corresponding letter on the Answer Sheet with a single line through the center.

Animals can perform many useful and entertaining jobs. People realize animals may not have the same intelligence as human beings. They are clever enough to learn many things. For example, dogs are particularly valuable in guiding the blind, protecting property, finding lost people, and hunting animals. It is not very difficult to learn some easy commands. Such as "sit, lie down, stay here, come here, bring clothes" and so on. Training a dog to be a watchdog often produces unexpected results. Some dogs quickly learn the difference between strangers and friends. This is because their masters welcome friends and invite them into their houses. Sometimes dogs always attack the postman who comes to deliver letters. One explanation for this behavior is that the postman comes to the house often. He never enters the house. So the dog thinks the postman is not wanted. Horses are also able to learn many things. Horses that are used for guard or police duty must learn never to be frightened of noises, traffic and other disturbances. Racing horses are able to run much faster than other horses.

Some dogs may become suspicious of postman because postman ______.