阅读理解 If half of the water were to be drained from the Pacific Ocean, a curious kind of submarine mountain called a guyot would be exposed. Guyots are strange formations that resemble mushroom stalks with flat tops. More than six hundred and fifty of these volcanic stalks have been discovered in the Pacific Ocean, and a few others have been discovered in the Atlantic sea plains.
It is easy to suppose that the guyots were formed by underwater lava spouts that piled up volcanic debris over the years, but just how they acquired their curiously flat tops remains a mystery. Shallow-water fossils found embedded(植入) in the tops of some guyots suggest that at one time the flat tops were much nearer the ocean's surface, but beyond this there is little that scientists can say.
One attempt to account for the flat tops suggests that the ocean levels were once much lower than they are today; thus wave action might have smoothed away the original peaks. Another theory holds that the guyots have probably always flat tops and that their weight has pushed them slightly toward the ocean floor, causing them to slowly submerge. But these are only theories—the guyots are still a geophysical puzzle.

单选题 The strange formations called guyots are actually________.

单选题 Scientist are puzzled by the guyots'________.

单选题 The discovery of shallow-water fossils indicates that the guyots were________.

单选题 The guyots are still a geophysical puzzle nowadays because________.

单选题 We can conclude from this passage that________.