You have been growing some animal cells in culture. The cells grow well for several weeks, and then their growth slows down. You conduct some tests and determine that there is a lot of lactic acid in the culture fluid. Which of the following is the most likely explanation for the poor growth of the cells?
a. There is too much glucose in the culture fluid.
b. There is not enough glucose in the culture fluid
c. There is too much oxygen in the culture fluid.
d. There is not enough oxygen in the culture fluid

The presence of oxygen is required by living beings to continue the process of aerobic cellular respiration which in turn generates energy by oxidation of nutrients. Here, oxygen serves as a terminal electron acceptor during electron transport from NADH and FADH2 which are formed during glycolysis and Kreb's cycle. When oxygen is not available to accept the electrons, cells cannot continue the process of aerobic cellular respiration.

Cells enter the alternative pathway of energy production which is the fermentation. During lactic acid fermentation, pyruvate formed by glycolysis accepts an electron from NADH. The result is the formation of lactate and NAD+. This recycling of NAD+ ensures the continuation of glycolysis to produce ATP molecules.

Accumulation of lactic acid in the culture fluid indicates the fact that the cells are not getting enough oxygen to perform aerobic cellular respiration and have entered lactate fermentation.