Read the following passage and mark the letter A, B, C, or D to indicate the correct answer to each of the questions.

There are national parks around the world, and in our country there are more than 30 national parks. A national park is a special area for the protection of the environment and wildlife.

National parks are wonderful playgrounds and lots of people visit them every year. There are places where you can camp without charge, or you can rent a room in a guest house. You can take long walks in the forests, take boat trips, or climb mountains. You are not allowed to hunt in the parks, so there are many wild animals. You can swim in the streams of most of the national parks. The park keepers sometimes go with the visitors on walks to tell them about the animals, plants, and mountains. They also have programmes and talks at the campgrounds and in the guest house so that people can learn all about the park and the things that are in it.

National parks play a key role in saving endangered species as well as protecting the environment and natural resources. It also helps raise the awareness of local residents about the importance of nature.

The word “them” in the passage refers to _____________. A. parks B. visitors C. park keepers D. animals