题目内容：

1.If there are people in the room, you're supposed _______ on the door before entering.

2.After that, Chinese started the tradition of ________ the moon and sharing the beautiful story.

3.David often ________ his friends' homes and chats with their parents.

4.When the little boy came in, the doctor _________ a dog's eyes.

5.In the future, humans _________ more secrets of the nature.