Space-time will be flat if

Space-time will be flat if
 
  a. the universe is expanding.
  b. the universe is accelerating.
  c. the average density of the universe is equal to the critical density.
  d. the universe is finite.
  e. the universe is homogeneous and isotropic.

Question 2

What type of geological activity is seen on Neptune's moon Triton?
 
  a. Plate tectonics and earthquakes are a common occurrence on Triton.
  b. Volcanoes of molten lava, similar to Earth, have been seen there.
  c. Geysers of liquid nitrogen are seen erupting from its surface.
  d. Geysers of liquid water are seen erupting from its surface.

Question 3

The cosmological principle states that the universe is I. expanding. II. accelerating. III. homogeneous. IV. isotropic.
 
  a. I and II
  b. III and IV
  c. I and III
  d. II and IV
  e. I, II, III, and IV

Question 4

Why is it believed that the rings of Saturn could not have formed just as the planet itself formed?
 
  a. Over time the solar wind should have blown away the particles in the rings.
  b. The particles in the rings should eventually fall into the planet in a short period of time.
  c. The rings are made of ice particles which would have been vaporized by the heat released as Saturn formed
  d. The outer planets do not have enough mass to hold on to the rings.