40. What does the last sentence in paragraph 4 mean?
1. Astronomers have not returned to Earth yet.
2. Astronomers have never landed on any Earth-size planet yet.
3. Astronomers have not found any planet with water, and thus life, yet.
4. Astronomers have not found any Earth-size planet where conditions are temperate for water,
and thus life, yet.
5. Astronomers have not found the so-called Goldilocks zone of a star where conditions are
temperate for water, and thus life, yet.