Which of the following best explains why people should not swim, hike, bike, or skate alone?

The importance of having someone present while engaging in activities such as swimming, hiking, biking, or skating stems from the need for safety. In case of an emergency, having another person nearby can make a significant difference. Whether it's dealing with an injury, a sudden medical issue, or an accident, having a companion ensures that help can be summoned quickly and efficiently. This can include alerting authorities, providing first aid, or simply offering support until assistance arrives.

Though the buddy system is a related concept, the primary focus here is on the immediate presence of another person to respond to emergencies. Other activities or rules may promote safety and provide guidance, but the essence of the question highlights the necessity of having someone with you for emergency situations specifically.