C. The mother dolphin and its baby were able to con with each other because

Решение:

В тексте описывается история: "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." Это означает, что они использовали специальное оборудование для связи.

Ответ: 3) they used special equipment.