单项选择题

Diving is popular all over the world. It is usually done in the ocean, but divers also explore other bodies of water, including lakes, rivers, and ponds. Snorkeling (戴呼吸管潜泳) on the surface (or just below) is a common form of diving, but many people use scuba (水肺), which stands for self-contained underwater breathing apparatus. Scuba divers carry a tank of air that allows them to breathe while diving underwater.
Throughout history, people have been fascinated by life underwater, and the Professional Association of Dive Instructors (PADI) estimates that there are now 6 million active divers worldwide. They engage in many different types of diving, of which wreck, cave, commercial, and military diving are just a few. The most common form of diving is sport diving, or recreational diving, which is practiced at depths of less than 130 ft (39 m). From these depths, divers can make a straight ascent to the surface. Diving beyond this limit requires advanced training. Because popular dive sites such as coral reefs and wrecks are typically not near land, most diving is done from boats. In some locations, however, divers can enter the water from shore. On a typical outing, the divers decide beforehand how long they will remain underwater and how deep they will descend. While the divers are underwater, at least one person serves as a spotter by remaining on the boat or on shore. All groups, whether diving from a boat or from shore, are required to fly a diver down flag (a red flag with a white diagonal slash) to alert boaters that people are underwater.

According to the sentence "They engage in many different types of diving, of which wreck, cave, commercial, and military diving are just a few." which of the following explanations is correct（）

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