2014年4月托福考试模拟试卷及答案(第四套)_第18页

　　When we accept the evidence of our unaided eyes and describe the Sun as a yellow star, we have summed up the most important single fact about it-at this moment in time. It appears probable, however, that sunlight will be the color we know for only anegligibly small part of the Sun's history.

　　Stars, like individuals, age and change. As we look out into space, We see around us starsat all stages of evolution. There are faint blood red dwarfs so cool that their surface temperature is a mere 4,000 degrees Fahrenheit, there are searing ghosts blazing at 100,000degrees Fahrenheit and almost too hot to be seen, for the great part of their radiation is in the invisible ultraviolet range. Obviously, the "daylight" produced by any star depends on its temperature; today(and for ages to come) our Sun is at about 10.000 degrees Fahrenheit, and this means that most of the Sun's light is concentrated in the yellow band of the spectrum, falling slowly in intensity toward both the longer and shorter light waves.

　　That yellow "hump" will shift as the Sun evolves, and the light of day will change accordingly. It is natural to assume that as the Sun grows older, and uses up its hydrogen fuel-which it is now doing at the spanking rate of half a billion tons a second-it will become steadily colder and redder.

　　In line 15, to which of the following does "it" refer?

　　(A) yellow "hump"

　　(B) day

　　(C) Sun

　　(D) hydrogen fuel

　　Passage 20.

　　In Philadelphia, for example, with a population of 150,000, people still had to go to the post office go get their mail. The confusion and congestion of individual citizens looking for their letters was itself enough to discourage use of the mail. It is no wonder that, during the years of these cumbersome arrangements, private letter-carrying and express businesses developed. Although their activities were only semilegal, they thrived, and actually advertised that between Boston and Philadelphia they were a half-day speedier than the government mail. The government postal service lost volume to private competition and was not able to handle efficiently even the business it had.

　　The word "they" in line 15 refers to

　　(A) Boston and Philadelphia

　　(B) businesses

　　(C) arrangements

　　(D) letters