How did King and Janik discover that dolphins use whistles to identify each other?
A
They compared the recordings of individual dolphin whistles to recordings of birdcalls.
B
They recorded individual dolphin whistles in the wild and studied them in their laboratory.
C
They followed the same dolphin group for years until the individual dolphins trusted humans.
D
They played the recordings of unique whistles to dolphins and listened as individual dolphins responded.
B
They recorded individual dolphin whistles in the wild and studied them in their laboratory.