A、Explorers. B、Doctors. C、Teachers. D、Coal-miners. A

The year is 2501, and the Earth is desperately short of fuel. All Earth’s coal, oil and wood have been used up long ago, and scientists have been sent to the ends of the universe in search of alternative forms of energy.
    A rich and powerful civilization has been discovered on the planet Jupiter, and Earth is supplied with fuel by constant supply fleets from that planet, but the price is very high. Unless it discovers an alternative source of energy soon, civilization on Earth will fail, and its inhabitants will have to move to other planets where they will be little more than slaves. Then Jupiter will take possession of Earth-- the most beautiful, most desirable planet in the universe.
    Human scientists have been sure for a long time that Jupiter possesses a method of creating constant energy and now ten explorers on a secret mission from Earth have discovered the secret--a kind of heavy, green dust which will change salt water to high grade petroleum. This dust covers a large part of Jupiter’s surface, and just a few pounds of it could solve the world’s energy crisis since it would reproduce itself continuously in Earth’s atmosphere.