ANS:B - not soluble in blood at high pressure.

In the diving apparatus, helium is used along with oxygen primarily because it is not soluble in blood at high pressure. When divers descend to great depths, they experience increased pressure, which can lead to a condition known as nitrogen narcosis if they breathe air containing a high concentration of nitrogen. Nitrogen narcosis, also known as "rapture of the deep," is caused by the narcotic effect of nitrogen under pressure, which can impair divers' judgment and coordination, similar to the effects of alcohol. Helium, on the other hand, does not cause nitrogen narcosis because it is not narcotic. Additionally, helium is less soluble in blood than nitrogen, which means that it is less likely to cause decompression sickness, also known as "the bends," when divers ascend to the surface. Therefore, the primary reason helium is used in diving apparatus along with oxygen is because it is not soluble in blood at high pressure, reducing the risk of nitrogen narcosis and decompression sickness.