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

From the earliest times the sea has been a place of secrets—a place of mysteries hidden from as by thousands of kilograms of water. The early Greeks sailed the oceans. They wrote about the seas. But since they could not dive deep 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 animals may help as 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 oceans. For if the sea dies, so dies life on earth. The sea may even become our home someday. People are studying ways to build cities on the ocean, and make more room to live in an overcrowded world.
    Modern study was done, at first, by divers. But it was too dangerous for divers to keep diving and coming to the surface.
    So now, laboratories on the oceans floor are the homes for divers for months on end. Divers can work outside in the water and returned to the laboratory to eat and sleep.
    Little by little we are solving the mysteries of the deep. Little by little the sea is showing us how we can live a better life on land.