Which of the Following Conditions Lead You to See an Absorption

Which of the following conditions lead you to see an absorption line spectrum from a cloud of gas in interstellar space?

A) The cloud is cool and lies between you and a hot star.
B) The cloud is extremely hot.
C) The cloud is visible primarily because it reflects light from nearby stars.
D) The cloud is cool and very dense, so that you cannot see any objects that lie behind it.