____________________ formed as Mercury cooled and shrank. (Fill in the blank)

____________________ formed as Mercury cooled and shrank.
 
  Fill in the blank(s) with correct word

Question 2

The energy from an AGN is produced by
 
  a. the collision of two spiral galaxies.
  b. the collision of two elliptical galaxies.
  c. the collision of two radio jets.
  d. matter flowing into a supermassive black hole.
  e. supernovae.

Question 3

____________________ are great lava flows covering 17 of the lunar surface.
 
  Fill in the blank(s) with correct word

Question 4

What type of galaxies do astronomers believe to be the most common?
 
  a. Elliptical
  b. Spiral
  c. Irregular
  d. Astronomers have no idea which type of galaxy is more common.