The krebs cycle is a stage of cellular respiration and occurs in aerobic organisms. In this case, we can say that the Krebs Cycle only occurs in the presence of oxygen, being essential for the completion of cell respiration.
The Krebs cycle only occurs after the completion of glycolysis, as it needs to be initiated by pyruvate, which is a molecule resulting from glycolysis. In this case, in addition to oxygen, the presence of pyruvate is essential for the krebs cycle to occur.
The difference would be that Mercury has a denser, warmer atmosphere and Earth has a not that dense and gassy atmosphere. The Earth's atmosphere also has a lot of layers while Mercury's doesn't. Those would be the reasons for them both being different. Hope this helped!
B is referring to cell theory, which basically states that all living things are made of cells and all cells come from other cells withe the exception of the first life forms.
