What do you do if you’re stranded （处于困境的）on a lonely island？Surely you need to find a way to get  in touch with the outside world. Your best chance of Body signals（信号）.
Pilots from different countries understand body signals if they have the correct training. When you wave  your arms up and down in a straight line，it means “yes”. When you point downwards and swing your arm from  side to side ，it means“no”. If you want the pilot to know that it’s safe to land，push your hands out in front of  you and bend（弯曲）your knees. If you want to say that it’s not safe to land ，put your arms in the air and move  them to one side.
Smoke signals
The smoke from a fire can be seen from far away ，so it’s a good way to draw attention .If  the weather is dry，it isn’t hard to start a fire. Remember that fires can be very dangerous if they get out of  control. Never  light a fire unless you’re sure that it can’t spread .If the ground is dark，light smoke can be seen more  easily .Green grass and leaves produce light smoke.
Ground-to-air signals
It’s a good idea to build some signals，too. Use large pieces of wood to make the symbols（标记）.If you can’t find any wood，use earth. Some useful symbols are：
F  “I need food and water”.
II  “I need medicine”.
I  “I am badly hurt”.
X  “I am unable to move from here.”
Pilots’ replies
If the pilot lowers the plane’s wings from side to side ，this means“message received and understood”（At  night，the pilot flashes the plane’s green lights.）If the pilot flies the plane in a clockwise circle ，this means“message  received but not understood”.（At  night ，the pilot flashes the plane’s red lights.）
However，there is no signal which means “message not received”. Why not？
Which picture means“it’s not safe to land”？

What can we learn from the passage？

A．Swinging your arm from side to side means“yes”.

B．If the ground is dark，light smoke can’t be seen.

C．If you need some medicine ，make a symbol “II”.

D．The pilots make replies with a blue light at night.

What is the passage mainly about？

A．The explanation of building signals on an island .

B．The importance of building signals on an island.

C．Ways of sending out signals on a lonely island.

D．Ways of receiving signals from a lonely island.