关于声现象，下列说法正确的是      （）

Mr Brown was going away for a week. Before he left, he said to his son, "if anyone asks for me, you can tell him that your father has been out for doing something, and will be back in a week, then be sure to ask him to sit down for a cup of tea."
"OK, Dad," said his son. But he was afraid his son couldn’t remember this, he wrote these words down on a piece of paper and gave it to him. His son put it into his small pocket, took it out and looked at it every now and then.
Four days passed, but no one came to see his father. The boy thought that there was no man to come and that the piece of paper was of no more use for him, so he burnt it that evening.
The next afternoon, someone knocked at the door. The boy opened it. A man was standing at the door and said, "Where is your father?" The boy put his hand into his pocket at once and looked for the piece of paper. He could not find it. He suddenly remembered he had burnt it, so he shouted, "No more."
The man was very surprised. He asked, "No more? I met your father last week. When did it happen?"
"Burnt yesterday evening."