A Molecular Biologist Is Interested in Purify a Recombinant Protein

A molecular biologist is interested in purify a recombinant protein by His-tagging,but the protein and vector both lack histidine residues.In this case,the molecular biologist could use which of the following technique(s) to acheive purification?

A) Add a 6xHis-tag to the C-terminus of the protein.
B) Add a series of histidine residues to the N-terminus of the protein.
C) Amplify the histidine-encoding sequence by PCR and add it to the gene of interest.
D) All of the choices are correct.