Read the following passage, then answer the questions:
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. lt appears probable, however, that sunlight will
be the colour 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 sunlight 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.
Choose the correct answer from a, b, c or d:
Why are very hot stars referred to as "ghosts"?
