Must we see both spectral lines splitting to find a spectroscopic ...

[charchew]

Must we see both spectral lines splitting to find a spectroscopic binary?
 
  What will be an ideal response?

Question 2

Most of the deuterium formed right after the Big Bang
 
  A) broke down into electrons and neutrons.
  B) was found in the globular clusters.
  C) turned into dark matter.
  D) is still around today.
  E) quickly burned into helium nuclei.

[mathjasmine]

Answer to Question 1

No, in some cases the second object is much fainter, but its gravity still causes the primary to move back and forth as both orbit their common center of mass.

Answer to Question 2

E