Analyze the endosymbiont theory of the evolution of plant cells by completing the sequence chart

The endosymbiont theory is an evolutionary theory that describes the origin of important characteristics of the eukaryotic cells. The theory was first introduced by the Russian botanist Konstantin Mereschkowski in 1905. According to the endosymbiont theory, two of the most defining characteristics of the eukaryotic cells, the mitochondria and the chloroplasts, evolved through symbiosis of individual single-celled prokaryotes.
The first step of the symbiosis is the formation of the nucleus and the endoplasmatic reticulum in the proto-eukaryotic cell, through the infolding of the plasma membrane.
The second step is the evolving of bacteria into mitochondria.
The third step is the evolving of the photosynthetic bacteria into chloroplasts.
Finally, the modern photosynthetic eukaryotic plant cell with the necessary organelles for photosynthesis (chloroplasts) is formed.

The right answer is c.sandstone.