A、They can be bred in any conditions. B、They produce oxygen. C、They can easily be carried in spaceships. D、They are organisms. B

If the population of the Earth goes on increasing at its present rate, there will eventually not be enough resources left to sustain life on the planet. By the middle of the 21st century, if present trends continue, we will have used up all the oil that drives our cars, for example. Even if scientists develop new ways of feeding the human race, the crowded conditions on Earth will make it necessary for us to look for open space somewhere else.
    An American scientist, Professor Carl Sagan, believes that before the earth’s resources are completely exhausted, it will be possible to change the atmosphere of Venus and so create a new world almost as large as Earth itself. The difficulty is that Venus is much hotter than the Earth and there is only a tiny amount of water there.
    Sagan proposes that algae, an organism that can live in extremely hot or cold atmospheres and at the same time produce oxygen, should be bred in conditions similar to those on Venus. As soon as this has been done, the algae will be placed in small rockets. Spaceships will than fly to Venus and fire the rockets into the atmosphere. In a fairly short time, the algae will break down the carbon dioxide into oxygen and carbon.
    When the algae have done their work, the atmosphere will become cooler but before man can set foot on Venus, it will be necessary for the oxygen to produce rain. The surface of the planet will still be too hot for men to land on it but the rain will eventually fall and in a few years something like Earth will be reproduced on Venus.