Many people believe that dogs are human's best friends. They are friendly, loyal, and can help people in many ways. But do you know dogs can also help with mental health? Studies show that playing with or even just looking at a dog can make people feel relaxed. It can lower stress and make people happier. For this reason, more and more hospitals and schools are using "therapy dogs" to help patients and students. These dogs are trained to stay calm and kind around strangers. They do not get nervous easily. When patients feel sad or worried, the dogs can cheer them up. For students who are under great pressure before exams, staying with a dog for a short time can help them calm down. Some people even keep dogs at home to improve their own mood. Taking a dog for a walk also gives people a chance to exercise and enjoy fresh air. In this way, dogs not only bring joy but also help people stay healthy. Truly, dogs are more than just pets. They are our helpers and our close friends.What is the main idea of the passage?A. Why dogs like staying with humans.B. Dogs are helpful to people's mental health.C. How to train therapy dogs.D. How to keep dogs healthy.