Which sentence in the passage uses definition to help explain the meaning of the word Celts?
Halloween

Halloween is one of the oldest celebrations in the world. In fact, it dates back to before Christianity began. Today, Halloween mainly is celebrated by wearing scary costumes and asking for treats. However, its primary purpose was to remember the dead. It was said to be a day when the spirits would cross over to the next world. Also, the day was originally used to mark the beginning of winter. This tradition was followed by the Celts. This ancient group of people lived in the Bronze Age. Halloween, for them, was rather glum and not a day for rejoicing. Over the years, the holiday became less grim. People began seeing it as a day of celebration. Today, families carve pumpkins to decorate their houses. They also play games, such as bobbing for apples.