阅读下面短文，从下面每小题的A、B、C、D四个选项中选出最佳选项。
What do you do if you are 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 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 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. 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 marks. If you can't find any wood, use earth. Some useful marks are:
F "I need food and water."
Ⅱ "I need medicine."
Ⅰ "I am badly hurt."
X "I am unable to move from here."
Pilot's 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 planes 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".
