Q:Read the following article and select the best answer. 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. Question: Why some elephants' call is called "secret language"? - Because it can't be heard by human ears. - Because it can only be heard by female elephants. - Because people don't know its meanings. - Because people haven't studied it completely.
A:
Because it can't be heard by human ears.