A population of cells is treated with a polypeptide hormone that induces mitosis and the hormone's ...

A population of cells is treated with a polypeptide hormone that induces mitosis and the hormone's cell surface receptor is isolated after 15 minutes and subjected to fragmentation and mass spectometry.
 
  Several of the peaks show a slight shift as compared to the same protein from cells not treated with the hormone. What is the most likely explanation of this phenomenon?
 
 A. Phosphorylation of some amino acid residues
  B. mRNA editing resulting in a slightly different amino acid sequence.
  C. Alternative exon usage.
  D. Proteolytic processing.