单选题     Scattered through the seas of the world are billions of tons of small plants and animals called plankton (浮游生物). Most of these plants and animals are too small for the human eye to see. They drift about lazily with the currents, providing a basic food for many larger animals. Plankton has been described as the equivalent of grasses that grow on the dry land continents, and the comparison is an appropriate one. In the potential food value, however, plankton far outweighs that of the land grasses. One scientist has estimated that while grasses of the world produce about 49 billion tons of valuable carbohydrates (碳水化合物) each year, the sea's plankton generates more than twice as much.
    Despite its enormous food potential, little effort was made until recently to farm plankton as we farm grasses on land. Now, marine scientists have at least begun to study this possibility, especially as the sea's resources appear even more important as a means of feeding an expanding world population.
    No one yet has seriously suggested the 'planktonburger' may soon become as popular as hamburger around the world. As a possible farmed supplementary food source, however, plankton is gaining considerable interest among marine scientists.
    One type of plankton that seems to have great harvest possibility is a tiny shrimp-like creature called krill (磷虾). Growing to two or three inches long, krill provide the major food for the giant blue whale, the largest animal on Earth. Realizing that this whale may grow to 100 feet and weigh 150 tons at maturity, it is not surprising that each one consumes more than one ton of krill daily. Krill swim about just below the surface in huge schools sometimes miles wide, mainly in the cold Antarctic. Because of their pink color, they often appear as a solid reddish mass when viewed from a ship or from air. Krill are very high in food value. A pound of them contains about 460 calories-about the same as shrimp or lobster, to which they are related.
    If the krill can feed such huge creatures as whales, many scientists reason, they must certainly be candidates as a new food source for humans.

单选题     Which of the following statements best describes the organization of the passage? ______

单选题     According to the passage, why is plankton considered to be more valuable than land grasses? ______

单选题     Why does the author mention 'planktonburgers' in paragraph 3? ______

单选题     What is mentioned as one distinguishing feature of krill? ______

单选题     The author mentions all of the following as reasons why plankton could be considered a human food source EXCEPT that it is ______.