Signs carry messages. For example, a sign at the bus stop helps you to know     (哪一个) bus to take. A sign on the wall of your school helps you find the     (图书馆). Signs through doors tell you where to go in or out.      (假如) you see the sign “No photography”, you can’t take      (照片). When you find the sign “ Danger”, you must be      (小心的). If you want to cross the road, you should      (等候) for the green light at the crossing. When you visit a museum and see a sign “Keep quiet”, you aren’t      (允许) to speak loudly. Have you      (曾经) noticed that there are a lot of signs around you and then you receive messages from     (它们) all the time. It’s necessary to      (记住) some of the signs.