Our Current Understanding of the Origin of Eukaryotic Organelles Such

Our current understanding of the origin of eukaryotic organelles such as mitochondria is that they

A) were copies of a cell nucleus that failed to be separated by cytokinesis.
B) are prokaryotes that were taken into a cell and now live there symbiotically.
C) are variations of the plasma membrane.
D) are new forms of life that arose inside other cells.