Elephants are able to differentiate between ethnicities  and sexes, and can tell an adult from a child --- all from the sound of a human voice. This is according to a study in which researchers played voice recordings to wild African elephants. The animals showed more fear when they heard the voices of adult Masai men. Livestock-herding Masai people do come into conflict with elephants, and this suggests that animals have adapted to specifically listen for and avoid them. The study is published in Proceedings of the National Academy of Sciences. Prof Karen McComb and Dr Graeme Shannon from the University of Sussex led the study. They explained that in previous research they had used similar playback experiments to show that elephants could tell --- from the sound of a lion's roar --- whether the animal was a female or a more dangerous male. Prof McComb wanted to find out if the animals used their very sharp sense of hearing to identify a potential danger from humans. The scientists recorded Masai men, women and children saying, in their own language, "Look, look over there, a group of elephants is coming". They also recorded Kamba men saying this phrase. While cattle-herding Masai people often come across free-ranging elephants, which can result in violent conflict, the Kamba people's more agricultural lifestyle does not generally bring them into aggressive touch with the animals. When the team played recordings of these different voices through a hidden loudspeaker, they found that elephant family groups reacted more fearfully in response to the voice of a Masai man, than to a Kamba man's voice. And the adult male Masai voices encouraged far more of these defensive reactions than the voices of women or boys. How did the researchers get to know the elephants' special ability of identification?
By observing the elephants in the zoo. By playing voice recordings to them. By recording the behaviours of elephants. By communicating with them in a special way.
Question: Elephants are able to differentiate between ethnicities  and sexes, and can tell an adult from a child --- all from the sound of a human voice. This is according to a study in which researchers played voice recordings to wild African elephants. The animals showed more fear when they heard the voices of adult Masai men. Livestock-herding Masai people do come into conflict with elephants, and this suggests that animals have adapted to specifically listen for and avoid them. The study is published in Proceedings of the National Academy of Sciences. Prof Karen McComb and Dr Graeme Shannon from the University of Sussex led the study. They explained that in previous research they had used similar playback experiments to show that elephants could tell --- from the sound of a lion's roar --- whether the animal was a female or a more dangerous male. Prof McComb wanted to find out if the animals used their very sharp sense of hearing to identify a potential danger from humans. The scientists recorded Masai men, women and children saying, in their own language, "Look, look over there, a group of elephants is coming". They also recorded Kamba men saying this phrase. While cattle-herding Masai people often come across free-ranging elephants, which can result in violent conflict, the Kamba people's more agricultural lifestyle does not generally bring them into aggressive touch with the animals. When the team played recordings of these different voices through a hidden loudspeaker, they found that elephant family groups reacted more fearfully in response to the voice of a Masai man, than to a Kamba man's voice. And the adult male Masai voices encouraged far more of these defensive reactions than the voices of women or boys. How did the researchers get to know the elephants' special ability of identification?
A. By observing the elephants in the zoo.
B. By playing voice recordings to them.
C. By recording the behaviours of elephants.
D. By communicating with them in a special way.
Answer: