One Sunday, Mark decided to go sailing in his boat with his friend Dan, but Dan happened to be away. Dan’s brother John offered to go instead though he did not know anything about sailing. Mark agreed and they set out to sea. Soon they found themselves in a thick fog. Mark was sure they would be hit by a big ship. Fortunately he saw a large buoy (浮标) through the fog and decided to tie the boat to it for safety. As he was getting onto the buoy, however, he dropped the wet rope. The boat moved away in the fog carrying John, Who did not know how to use the radio. He drifted (漂流) about and was not seen until twelve hours later. Mark spent the night on the buoy. In the early morning he fell asleep. He was having a bad dream when a shout woke him up. A ship, the Good Hope, came up and he climbed onto it and thanked the captain. The captain told him that John had been picked up by another ship and the ship’s captain had sent out a message. “Without the message I would not have found you on the buoy,” he said.（1）Why didn’t Mark and Dan go sailing together?