Question 11 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.

11. What is the speaker mainly talking about?

解析：C。短文开头提及人们喜欢破译密码，引出二战中从未被破译的Navajo密码，之后短文对这种密码的编译、使用者、使用方法以及战后美国政府对密码人员的奖励进行介绍。因此可知，文章主要从Navajo密码展开，全面讲述了这种从未被成功破译的密码，因此选择C项。