In this task, you're given an article and an answer. Your task is to generate the question for the answer based on the given article.

Let me give you an example: Article: There are a lot of public libraries in Britain. These libraries have more than one hundred and fifteen million books, not including those libraries of public schools. Public libraries not only lend books, music records but also help children, patients in hospitals and others to get special play readings, film shows and so on. The first public libraries were founded in about the year 1850. Today's public libraries usually have a reading-room, a lending room and a reference room. The greatest and most famous library in Britain is that of the British Library in London. It was founded in 1973 and has over 10, 000, 000 books. It has nearly every magazine, newspaper and book published in Britain and other parts across the world.
Answer: 10,000,000 or less.
The answer to this example can be: How many books are there in the British Library?
Here is why: In the Article, 10,000,000 is mentioned for the number of books available in the British Library, so this is a good question.

OK. solve this:
Article: Researchers at San Diego Zoo have been studying what has been described as the "secret language" of elephants. They have been monitoring communications between animals that cannot be heard by human ears. The elephant's call will be familiar to most people, but the animals also give out growls . Their growls, however, are only partly audible ; two-thirds of the call is at frequencies that are too low to be picked up by our hearing. To learn more about the inaudible part of the growl, the team attached (......) a microphone sensitive to these low frequencies and a GPS tracking system to eight of the zoo's female elephants. The researchers could then relate the noises the animals were making to what they were doing. Matt Anderson, who led the project, told BBC News, "We're excited to learn how they interact and contact with one another." The team has already learned that pregnant females use this low frequency communication to announce to the rest of their long gestation of over two years, in the last 12 days we see the low part of the growl, which we can't hear. This we believe is to announce to the rest of the herd that the baby is upcoming," said Dr Anderson. The researchers believe that this also warns the elephants to look out for coming danger. "You may think that a baby calf of about 300 pounds would not be as open to predation as other species," he says. "But packs of hyenas are a big threat in the wild." Female elephants are only in season for around four days every four years and these calls can be heard by males more than two miles away.
Answer: Because it can't be heard by human ears.
Answer:
Why some elephants' call is called "secret language"?