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Thus, many people in the early nineteenth century thought that when a ship sank, it never reached bottom, but instead floated on this layer of dense water.
In 1841 Edward Forbes was offered the chance to serve as naturalist aboard HMS Beacon, an English Royal Navy ship assigned to survey the Aegean Sea. For a year and a half the Beacon crisscrossed the Aegean waters. During that time Forbes was able to drag his small, triangular dredge-a tool with a leather net for capturing creatures along the sea bottom-at a hundred locations, at depths ranging from 6 to 1,380 feet. He collected hundreds of different species of animals, and he saw that they were distributed in eight different depth zones, each containing its own distinct assemblage of animal life, the way zones of elevation on the side of a mountain are populated by distinct sets of plants.
Forbes also thought he saw, as he later told the British Association, that "the number of species and individuals diminishes as we descend, pointing to a zero in the distribution of animal life as yet unvisited. This zero, Forbes casually speculated-he simply extended a line on his graph of animal number versus depth-probably began at a depth of 1,800 feet. Below that was the final zone in Forbes's scheme, zone nine, a zone that covered most of the ocean floor and thus most of the solid surface of Earth: Forbes called this the azoic zone, where no animal, to say nothing of plants, could survive.
Forbes's azoic zone was entirely plausible at the time, and it was certainly far from the strangest idea that was then entertained about the deep sea. In the first decade of the nineteenth century, a French naturalist named François Péron had sailed around the world measuring the temperature of the ocean. He found that the deeper the water, the colder it got, and he concluded that the seafloor was covered with a thick layer of ice. Péron ignored the fact that water expands when it freezes and that ice therefore floats. A more popular belief at the time was that water at great depth would be compressed to such a density that nothing could sink through it. This ignored the fact that water is all but incompressible. But even the more sensible naturalists of the day were guilty of a similar misconception. They imagined the deep sea as being filled with an unmoving and undisturbable pool of cold, dense water. In reality the deep is always being refreshed by cold water sinking from above.
The central implication of all these misconceptions was that nothing could live in the abyss (deep), just as Forbes's observations seemed to indicate. But Forbes erred in two ways. One was the particular study site he happened to use as a springboard for his sweeping postulate of a lifeless abyss. Although the Aegean had been the birthplace of marine biology, its depths are now known to be exceptionally lacking in animal diversity. Moreover, through no fault of his own, Forbes was not particularly successful at sampling such life as did exist at the bottom of the Aegean. It was his dredge that was inadequate. Its opening was so small and the holes in the net so large that the dredge inevitably missed animals. Many of those it did catch must have poured out of its open mouth when Forbes reeled it in. His azoic zone, then, was a plausible but wild extrapolation from pioneering but feeble data.
As it turned out, the existence of the azoic zone had been disproved even before Forbes suggested it, and the theory continued to be contradicted regularly throughout its long and influential life. Searching for the Northwest Passage from the Atlantic to the Pacific in 1818, Sir John Ross had lowered his "deep-sea clam"-a sort of bivalved sediment scoop-into the waters of Baffin Bay(an inlet between the Atlantic and Arctic oceans), which he determined to be more than a thousand fathoms deep in some places. Modern soundings indicate he overestimated his depths by several hundred fathoms, but in any case Ross's clam dove several times deeper than Forbes's dredge. It brought back mud laced with worms, and starfish that had entangled themselves in the line at depths well below the supposed boundary of the azoic zone.
题干分析：待插入句“Thus”引导结果，说明前一句是“人们觉得船沉了不会触底”的原因。“this layer of dense water”说明前一句出现密度高的水。
C选项前“water at great depth would be compressed to such a density”就是待插入句的“this layer of dense water”。
C选项前“nothing could sink through it”就是待插入句“when a ship sank, it never reached bottom”的原因。