Question 10 is based on the passage you have just heard.

        (9) Many people enjoy secret codes. The harder the code the more some people would try to figure it out. In wartime, codes are especially important. They help army send news about battles and the size of enemy forces. Neither side wants its code broken by the other. (10) One very important code was never broken. It was used during World War II by the Americans. It was a spoken code, never written down and it was developed and used by Navajo Indians. They were called the Navajo code talkers. The Navajos created the code in their own language. Navajo is hard to learn and only a few people know it. So it was pretty certain that the enemy would not be able to understand the code talkers. In addition, the talkers used code words. They called a submarine and an iron fish and a small bomb thrown by hand a potato. If they wanted to spell something, they used code words for letters of the Alphabet. For instance, the letter A was ant or apple or ax. The code talkers worked mostly in the islands in the Pacific. One or two would be assigned to a group of soldiers. They would send messages by field telephone to the code talker in the next group. And he would relay the information to his commander. (10) The code talkers played an important part in several battles. They helped the troops coordinate their movements and attacks. After the war, the U.S. governments honored them for what they had accomplished. Theirs was the most successful wartime code ever used.

10. What do we learn about the Navajo code talkers?

解析：A。根据录音可知，在二战期间，美国的一种密码从未被敌方破译。这种密码以印第安部落纳瓦霍人的语言为基础发展而来，没有书面形式，只是在纳瓦霍之间口口相传。纳瓦霍破译员帮助军队协调行动和攻击，在战争中起了重要作用。因此A项正确。