阅读下面的短文，然后根据短文内容，在每小题所给的四个选项中选出最佳选项。

　　What do you do if you’re in trouble on a lonely island? Surely you need to find a way to get in touch with the outside world．Your best chance of doing this is to draw the attention of a passing plane．

　　Body signals

　　Pilots(people who drive planes)in 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(摆动)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, however, that fires can be very dangerous if they get out of control．Make sure if you light(点燃)a fire, it can't be spread(扩散)．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．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 makes plane's lights green)．If the pilot flies the plane in a clockwise circle, he means “message received but not understood”．(At night, the pilot makes the plane's lights red．)

　　However, there is no signal which means “message not received”．Why not?

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