A、Attracting mates. B、Staking out their territories. C、Hearing each other. D、Sleeping. D细节题。在文章的最后谈到嘈杂的社会环境给动物听到彼此的声音带来了麻烦。接着下面举

The world has become noisier place, say experts at the Library of Natural Sounds in Ithaca, New York. The library has the world’s largest collection of natural sound recording — including birds, frogs, and mammals. The recordings are used for scientific study, field and mammals.(16)The recordings are used for scientific study, field guides, and even some movie background sounds.(17)Finding quiet areas for recording is becoming difficult, says Greg Budney, head of the library. Even in sparsely populated areas, animal sounds can be submerged by moving trucks, highway traffic, or aircraft. Obtaining a clean recording of an animal may take days or even years, and a habitat recording of all the species in an area is virtually impossible.(18)Animals are having trouble hearing each other in loud noises as well. A report from the British Ecological Society claims that, because of noise from nearby British roadways,(18)some birds are having trouble attracting mates and staking out their territories.
Questions 16 to 18 are based on the passage you have just heard.
16. Which of the following functions of the sound recording is not mentioned?
17. What is becoming difficult to record according to Greg Budney, head of the library?
18. In the noisier world, animals also have troubles. Which of the following is not included?