【正确答案】 A

【答案解析】[听力原文]27-30
Sign language is a way of talking by using signs. Native Americans used sign language when they met strangers. In this way, they could find out whether the stranger was a friend or an enemy. In Native American sign language, signs were made with the hands. One sign meant "man". Another meant "horse". To tell the .time of day when something happened, a Native American pointed to the sky. He showed where the sun had been at the time.
Native Americans usually used signals when they wanted to send messages to someone far away. To make signals, a Native American might ride a small horse. He might use smoke, a mirror or fire arrows.
To signal that he had seen many animals, a Native American rode his small horse in a large circle. Sometimes the Native American gave a signal like this and then went away to hide. This meant that there was danger.
He could also send many signals with a mirror. He usually used the mirror to warn someone of danger or to get the attention of a person far way. But he also used it to send messages in code.
Of course, mirrors could only be used when the sun was shining. At night, Native Americans used fire arrows for signaling.

主题句后直接提到“Native Americans used sign language when they met strangers. In this way, they could find out whether the stranger was a friend or an enemy.”，所以答案是A。