[单选题]The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Choose the missing words of the phrase or sentence. Training a dog to be a watchdog often produces unexpected results. Some dogs quickly learn the difference between unwanted people and friends. This is because their masters welcome friends and invite them into their houses. However, some dogs will always attack the postman who comes to deliver letters.

Dogs are expected to be （） when trained.