Search and Rescue Dogs Read the following article:

Have you ever seen footage of search and rescue dogs working at disasters? Have you seen movies or TV programs in which dogs are used to find lost children or track escaped prisoners? Search and rescue dogs save many lives, but they also inspire respect and admiration. They go where we can't go, use senses that out-perform our own feeble versions, and they save lives with the help of their expert trainers and handlers.

Search and rescue dogs (SARs) are not just born, however. They require a huge investment of time, dedication, training, and patience almost from the day they are born. SAR dogs are carefully selected when they are quite young. Trainers look for dogs that lack aggression and respond well to commands. They also look for a certain mental focus—it can be on a type of play or a special toy—that will enable the dog to work at a single task for a long time. This focus is important to the training of the dog, but also will be required when the dog is searching for and following human scent.

If a dog is selected for SAR training, it will be taught—through a series of games—how to make the most of its powerful nose. For example, dogs learning to follow scents on the air might play a tug-of-war game with the trainer involving a special toy. Then with a helper restraining the dog, the trainer will run away with the toy, find a hiding spot, and then call the dog. The dog can follow the scent of both the trainer and the toy. In this way, the dog is practicing important skills even though it may seem like just a game. Eventually, the toy is removed from this game, and the dog focuses on the human scent. Later the dog will learn more difficult tasks: to pick up a scent from an article of clothing or pick out a particular human scent in a crowd of people, for example.

Read the third paragraph of the article again. Why is the toy removed from the training after a certain time?

Once trust has been established, it's not needed.
The dogs need to shift their focus to human scent.
The type of play needs to become more difficult.
Toys are too easily traced by their scent.