Intergalactic hydrogen clouds are easiest to study by looking at ...

[jlmhmf]

Intergalactic hydrogen clouds are easiest to study by looking at
 
  A) 21 cm radio emission.
  B) radio emission from molecular hydrogen.
  C) X-ray emission.
  D) absorption lines in quasar spectra.

Question 2

One fundamental consequence of the Copernican model which many people had difficulty accepting at the time it was proposed was that the Earth
 
  A) must be moving through space
  B) was rotating on its axis
  C) must be stationary in space
  D) must be smaller than the Sun

[itsakadoozi]

Answer to Question 1

D

Answer to Question 2

A

[jlmhmf]

Excellent

[vickybb89]

Gracias!