challenge: What is the composition of the prosecretin precursor protein?
  
  A. Only a spacer and a 72-amino acid C-terminal peptide
  B. Only the signal peptide and secretin itself
  C. Only the secretin peptide and a C-terminal peptide
  D. An N-terminal signal peptide, spacer, secretin itself, and a 72-amino acid C-terminal peptide
  Answer: 