A、They had no pollution. B、They had small boats. C、They could not think. D、They could not dive deep. D

From the earliest times the sea has been a place of secrets -- a place of mysteries hidden from us by thousands of kilograms of water. The early Greeks sailed the oceans. They wrote about the seas. But since they could not dive into the waters, they could not lay bare these secrets. And so the secrets remained.
    But today we have more ways to study the sea than did the early Greeks. Scientists are learning about life in the sea, and how it may improve life on land. The study of sea plants and sea animals may help us to learn new ways of getting food.
    Scientists are searching the ocean floor for oil and gas deposits to keep our world running smoothly. And they are also learning how not to pollute the ocean. For if the sea dies, so does life on earth. The sea may even become our home some day. People are studying ways to build cities on the ocean floor, and make more room to live in an overcrowded world. A modem study was done by divers. But it was too dangerous for divers to keep diving and coming to the surface.
    So now, laboratories on the ocean floor are the homes for divers for months on end. Divers can work outside in the water and return to the laboratories to eat and sleep.
    Little by little we are solving the mysteries of the deep sea. Little by little the sea is showing us how we can live a better life on land.