When we accept the evidence of our unaided (không được giúp đỡ) 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 probably, however, that sunlight will be the color we know for only a negligibly small part of the Sun's history. Stars, like individuals, age and change. As we look out into space, we see around us stars at 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 100,000 degrees 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. Question 31. What is the passage mainly about? A. Faint dwarf stars B. The evolutionary cycle of the Sun C. The Sun's fuel problem D. The dangers of invisible radiation Question 32. What does the word "it" in the last paragraph refer to ? A. yellow "hump" B. day C. Sun D. hydrogen fuel Question 33. Why are very hot stars referred to as "ghosts"? A. They are short- lived. B. They are mysterious. C. They are frightening. D. They are nearly invisible Question 34. Which of the following sentences is NOT true according to the passage? A. The Sun’s history is still mysterious. B. All the stars depend on the Sun for light. C. The Sun is too hot to be seen by naked eyes. D. The sun never uses up its hydrogen fuel.