Normal Protein Degradation by Posttranslational Modification Serves Which of the Following

Normal protein degradation by posttranslational modification serves which of the following functions in vivo?

A) to eliminate abnormal proteins that could become harmful at high concentrations
B) to remove enzymes that appear outside their normal cellular location
C) to permit the regulation of cellular metabolism by eliminating superfluous isoenzymes
D) all of the above