In 600 B.C., the Assyrian Empire had just fallen. At its height, it had extended from Egypt to Babylonia, for an extreme length of 1,400 miles. It was soon to be (B1) the Persian Empire, which extended from Cyrenaica to Kashmir, for an extreme length of 3,000 miles. Undoubtedly, the common (B2) of these empires had only the vaguest (B3) of the extent of the maim and were content to live and die on their own few （B4） , or, on some occasions, to travel from village to (B5) village. Travelers and soldiers, however, must have had some (B6) of the vastness of these empires and of the still greater vastness of what must lie beyond. There must have been in the ancient empires those who occupied themselves with what might be considered the first cosmological (宇宙论的)problem (B7) scholars: Is there an end to the Earth To be sum, no man in ancient times, however far he traveled ever came to any (B8) end of the Earth. At most, he reached the (B9) of an ocean whose limits were beyond the horizon. If he transferred to a ship and sailed outward, he never (B10) in reaching the end either. Did that mean there was no end The answer to that question depended on the general shape one (B11) for the Earth. All men, before the time of the Greeks, made the (B12) that the Earth was flat, as indeed it appears to be, （B13） the minor irregularities of the mountains and valleys. If any pre-Greek ancient thought otherwise, his name has not (B14) to us and the record of his thinking has not (B15) Yet if the Earth were indeed fiat, an end of some sort would seem an almost foregone (B16) . The alternative would be a flat suce that would go on forever and forever -- one that would be infinite in extent, in other words. This is a most uncomfortable concept; throughout history, men have (B17) to avoid the concept of endlessness in space and time as something impossible to (B18) and understand and therefore something that cannot easily be worked with or (B19) about. On the other hand, if the Earth does have an end -- if it is (B20) there are other difficulties. Would not people fall off if they approached the end too closely

A．concept
B．fancy
C．imagination
D．perception