Which of the following is the correct order of ligand field strength?

• A
  $S^{2-} < C_2O_4^{2-} < NH_3 < en < CO$
• B
  $S^{2-} < C_2O_4^{2-} < NH_3 < en < CO$
• C
  $NH_3 < en < CO < S^{2-} < C_2O_4^{2-}$
• D
  $S^{2-} < NH_3 < en < CO < C_2O_4^{2-}$