Q: 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.
Article: Dolphins call out the specific names of loved ones when they become separated, a study finds. Other than humans, they are the only animals known to do this, according to the study. The big difference with dolphins is that these communications consist of whistles , not words. Earlier research found that dolphins have a "signature whistle" that would be somewhat like human shouting, "Hey everybody! I'm an adult healthy male named George, and I mean you no harm. " The new finding is that dolphins also say the names of certain other dolphins. "Animals produced copies of the same sound when they were separated from a close partner. This supports our belief that dolphins copy another animal's signature whistle when they want to reunite with that animal," Stephanie King, who led the study, says. King and her colleagues collected acoustic data from wild dolphins around Sarasota Bay, Florida, from 1984 to 2009. The researchers also studied four adult male dolphins _ at the Seas Aquarium, also in Florida. Those males are adults that keepers named Calvin, Khyber, Malabar and Ranier, all of whom, as well as all of the wild ones, developed their own signature whistles that served as names in communication with other dolphins. "A dolphin uses its signature whistle to broadcast its identity and announce its presence. The signature whistles allow animals to identify one another over long distances and allow animals to recognize one another and to join up with each other," King explains. "Dolphin whistles can be heard up to 20km away, depending on water depth and whistle frequency ." The researchers said dolphins copy the signature whistles of loved ones, such as a mother, when the two are separated. These "names" are always used in positive situations, and are only directed toward loved ones. While researchers are often unwilling to apply the word language to non-human communications, dolphins clearly have a very complex communication system.
Answer: The dolphin's age.
A:
Which of the following plays a part in whether dolphin whistles can be heard over a distance?