Question 9 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.

9. What does the speaker say many people enjoy doing?

解析：C。根据短文开头可知，很多人喜欢密码。密码越难，他们就越想破译出来。因此C项正确。