Task: Question Answering
Input (English): Question: In the absence of oxygen, the mitochondrial citric acid cycle is inhibited because: 
 Options:  <1> Increases the concentration of ADP, an inhibitor of isocitrate dehydrogenase. <2> The phosphorylations at the substrate level can not be produced. <3> No pyruvate available. <4> ATP synthase is inhibited. <5> The necessary coenzymes can not be reoxidized.
Output (English): 5