Given a scientific passage and an answer, generate a question for the given answer.

[Q]: Passage: Kevin Walsh. Cows are able to digest grass with the help of the bacteria methanogens in their gut . CC BY 2.0. Answer: bacteria
[A]: A type of what in a cow's gut enables the animal to digest grass?

[Q]: Passage: Air movement takes place in the troposphere. This is the lowest layer of the atmosphere. Air moves because of differences in heating. These differences create convection currents and winds ( Figure below ). Answer: troposphere
[A]: What is the lowest layer of the atmosphere called?

[Q]: Passage: Then, about 2 or 3 billion years ago, oxygen was gradually added to the atmosphere by early photosynthetic bacteria. After that, living things could use oxygen to break down glucose and make ATP. Today, most organisms make ATP with oxygen. They follow glycolysis with the Krebs cycle and electron transport to make more ATP than by glycolysis alone. Cellular respiration that proceeds in the presence of oxygen is called aerobic respiration . Answer: aerobic respiration
[A]:
Cellular respiration that proceeds in the presence of oxygen is known as what?