Questions $94-96$ A tissue or cells can be mashed up in a blender to form a liquid homogenate and then spun in an ultracentrifuge at high speeds to separate it into layers based on differences in density. The densest cell organelles settle to the bottom of the tube, forming a pellet. The supernatant, the liquid above the pellet, which contains less-dense organelles, can be poured off and respun. In this way, cell organelles can be isolated. A homogenate of human tissue was processed and spun in this way. See the diagram below.
Pellet $\mathrm{B}$ was found to have the highest rate of oxygen uptake compared with the other pellets. Which organelles did it contain?
(A) Ribosomes
(B) Mitochondria
(C) Vacuoles
(D) Golgi
(E) Nuclei