At Each Step of a Protein Purification Scheme, Purity Should

At each step of a protein purification scheme, purity should be ascertained by, for example,
I. SDS-PAGE gel electrophoresis
II. specific activity of antibodies
III. western blotting
IV. volume measurement

A) I only
B) IV only
C) I, II and III
D) II, III and IV
E) I, III and IV