单选题 There was once a king. He wanted to send a message to a friend who lived far away, but he didn't want anyone else to see it. He wanted it to be a secret one. The king thought and then had an idea. He knew how he could do that.
The king called in a servant whom he trusted most and said to him, "I want you to shave all the hair off your head." And the servant did so. Then the king wrote something on his head. After that, the king said, "I want you to stay alone until your hair grows back. Your hair will hide what I have written. I'm going to put yon in a room for a month."
One month passed. The king let the servant out and asked him to send the message. He said, "I want you to show him the message on your head. Only he may see it."
When the servant got to the king's friend, he cut his hair off again to read the message. Then the servant washed his head, and the message was off.
No one but the king and his friend ever knew what the message said. It was a secret message between them. It might have been the first secret message ever sent.

单选题 The king wanted all his people to know the message.

单选题 The king asked a servant to help him to send the message.

单选题 The king asked the servant to send a letter in paper to his friend.

单选题 The king wrote the message on the servant's head.

单选题 The servant doesn't know what the message is about.