Which of the following statements is correct?
a) None of these statements are correct
b) The membrane-bound mucins are cross-linked like the secreted mucins
c) Mucins secreted by goblet cells are short lipopeptides
d) The free cysteine residues of the mucin polypeptides can be used to form covalent bonds with molecules of secreted IgA and defensins
e) All of these statements are correct