Using Protein Gel Electrophoresis to Measure Genetic Variation in Populations

Using protein gel electrophoresis to measure genetic variation in populations has some drawbacks. Which of the following is/are problems with the technique?

A) It can't detect synonymous mutations. That is, differences in DNA sequences that result in the same amino acid sequence can't be detected with this method.
B) It can't detect differences in noncoding DNA.
C) It can only be used with proteins that can be stained or otherwise labeled and visualized.
D) It can't detect mutations that do not affect the mobility of a protein within the gel, even if the protein's amino acid sequence is changed.
E) None of the answer options is correct.