Endosymbiosis is a theoretical process that may explain the origins of several organelles found in eukaryotic cells. In this theory, these organelles were once separate organisms that formed symbiotic relationships with other cells millions of years ago, leading to the cell structures we see today. Which of the following characteristics of mitochondria BEST supports the argument that mitochondria were once separate organisms?

A
Mitochondria produce chemical energy in the form of ATP.
B
Mitochondria have their own unique, double stranded DNA.
C
Mitochondria divide by pinching in half, similar to the process used by bacteria.
D
Mitochondria contain a double membrane and the inner membrane looks very similar to bacterial membranes.