Which Might Be Most Useful for Determining the Amino Acid

Which might be most useful for determining the amino acid residues in an enzyme's active site?

A) Use an irreversible inhibitor that reacts with the side chains of specific amino acids.
B) Determine the entire primary structure of the protein.
C) Use a competitive inhibitor and plot the rate vs concentration of inhibitor.
D) Unfold the enzyme with urea and 2-mercaptoethanol.Then incubate and crystallize it in the presence of the substrate followed by X-ray crystallography.