A、They could not think. B、They had no pollution. C、They could not dive deep. D、They had small boats. C短文中用转折表述提到But since they c

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 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 sea animals may help us learn new ways of getting food. Scientists are searching the ocean floor for oil and gas, to keep our world running smoothly. And they are also learning how not to pollute the oceans. 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, 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, labouratories on the ocean floor are the homes for divers for months on end. Divers can work outside in the water and return to the labouratory 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.
Questions 23 to 25 are based on the passage you have just heard.
23. Who wrote about the seas in early times?
24. What kept early people from understanding the sea?
25. What happens if the sea dies?