Which of the following is/are the causes of intergranular corrosion?

• A. Impurities at the grain boundaries
• B. Enrichment of one of the alloying elements at the grain boundary
• C. Depletion of one of the alloying elements at the grain boundary
• D. Impurities at the grain boundaries, enrichment, and depletion of one of the alloying elements at the grain boundary