C. The mother dolphin and its baby were able to communicate with each other because... 1) they were in the same tank. 2) they were relatives. 3) they used special equipment. 4) they were in Hawaii.

The text says: "The special call was made in an aquarium in Hawaii, where the mother and its two-year-old baby swam in separate tanks connected by a special underwater audio link." This means they were able to communicate using special equipment.