A、To stabilize world temperature. B、To secure the health of life system. C、To stop the marine creatures from dying. D、To restore

Fifty years ago, when I began exploring the ocean, [l6]no one imagined that we could do anything to harm the ocean by what we put into it or by what we took out of it. It seemed, at that time, to be a sea of paradise, but now we know that we are facing paradise lost.
    I want to share with you my personal view of changes in the sea that affect all of us, and to consider why it matters that in 50 years, we’ve lost—actually, we’ve taken, we’ve eaten—more than 90 percent of the big fish in the sea; why you should care that nearly half of the coral reefs have disappeared; [17]why a reduction of oxygen in large areas of the Pacific should concern not only the creatures that are dying, but it really should concern you. It does concern you, as well.
    There was still time. Well, now is that time. I hope for your help to explore and protect the wild ocean in ways that will restore the health and, in so doing, secure hope for humankind. Health to the ocean means health for us.
    For me, as a scientist, it all began in 1953 when I first tried underwater breathing device. It’s when I first got to know how fish swimming in mysterious ocean. I actually love diving at night; you see a lot of fish then that you don’t see in the daytime. Since then, I’ve started designing and building 13 underwater systems to access the deep sea. [18]I led a five-year National Geographic expedition, the Sustainable Seas expeditions, using these little submarines. They’re so simple to drive that even a scientist can do it. And I’m living proof.
    Over time, most of the planet’s organic carbon has been absorbed and stored there. The ocean drives climate and weather, stabilizes temperature, shapes Earth’s chemistry. Water from the sea provides home for about 97 percent of life in the world, maybe in the universe. No water, no life; no blue, no green.
    [19]A global plan of action with a world conservation union is underway to protect biodiversity, to recover from the impacts of climate change. New technologies are needed to map, photograph and explore the 95 percent of the ocean that we have yet to see. I wish you would use all means at your disposal—films, expeditions, the web, new submarines—and campaign to arouse public support for a global network of marine protected areas—hope spots large enough to save and restore the ocean, the blue heart of the planet.
16. What was beyond imagination when the speaker began to explore the ocean 50 years ago?
17. What should we worry about in the Pacific except for the dying creatures?
18. How long has the National Geographic expedition led by the speaker lasted?
19. What is the purpose for the global plan of action?