Secondary endosymbiosis occurs when

• A. a eukaryotic cell engulfs a cell that has already undergone primary endosymbiosis.
• B. a prokaryotic cell engulfs a cell that has already undergone primary endosymbiosis.
• C. a eukaryotic cell engulfs a cell that has not undergone primary endosymbiosis.
• D. a prokaryotic cell engulfs a cell that has not undergone primary endosymbiosis.