문제 내용: 내용의 흐름상 적절하지 못한 문장은?
  The earth is a planet full of life. One of the reasons for
  this is that our sun is the kind of star that can support
  life on a planet. All the time the sun continues to send
  out a steady supply of heat and light. For our sun is a
  stable star.
선택지 1: This means that it stays the same size.
  And its output of energy (heat and light) does not change
  much.
선택지 2: Some stars are not stable.
선택지 3: They grow bigger
  and hotter and then smaller and cooler.
선택지 4: Life
  could exist under these great changes. We are here because
  a steady amount of energy pours forth from our sun.