This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Techniques in molecular biology, Subject name: Biochemistry
Sanger's reagent is 
Select and explain. 
A. Dinitrobenzene, B. Dichlorobenzene, C. Tetra-nitrobenzene, D. Tetrachlorbenzene
A: Dinitrobenzene
Answer: A: Dinitrobenzene 
Explanation: Sanger's reagent or di-nitro-fluoro-benzene (DNFB) is used for polypeptide sequencing. Other methods of Polypeptide sequencing: Edman's degradation (Edman's reagent is Phenyl-isothiocyanate) - Determine sequence from N-terminal end and can sequence 30-60 amino acid residues. But it cannot work if N-terminal amino acid is chemically modified. Use of various chemical or enzymes to cleave peptides - method used for large polypeptides Mass spectrometry - 3 pas of mass spectrometer are ionization source, mass analyzer and ion detector.