Two heavenly bodies $S_1$ and $S_2$,not far from each other,are seen to revolve in orbits:

• A
  Around their common centre of mass
• B
  Which are arbitrary
• C
  With $S_1$ fixed and $S_2$ moving round $S_1$
• D
  With $S_2$ fixed and $S_1$ moving round $S_2$