Which gases does a Nitrox diving computer consider when computing allowable bottom time?

The correct choice identifies oxygen and nitrogen as the primary gases that a Nitrox diving computer considers when calculating allowable bottom time. In Nitrox diving, the gas composition typically used is a mixture of oxygen and nitrogen, where the proportion of oxygen is higher than what is found in standard air.

Nitrox has lower nitrogen content than regular air, which allows divers to extend their bottom time while reducing the risk of nitrogen narcosis and decompression sickness, commonly known as "the bends." By closely monitoring the partial pressures of both gases, a diving computer can effectively manage exposure limits and allow divers to dive safely within accepted guidelines.

The other gases listed, such as nitrous oxide, carbon monoxide, and helium, are not the primary focus in the context of Nitrox diving. Nitrous oxide and carbon monoxide are not components of breathing gases used during recreational diving. Helium is involved mainly when diving with trimix (a mix of oxygen, nitrogen, and helium) but is not relevant in the context of typical Nitrox diving computations. Thus, the gas combination of oxygen and nitrogen is crucial for understanding the limits of diving, making this the best answer.