Question

oxygen concentration in the air is considered deficient if it drops below 10.5% 15.5% 19.5% 23.5%

Explanation:

The normal oxygen concentration in air is about 20.9%. Oxygen is considered deficient when it drops below a certain level, and 19.5% is a recognized threshold for oxygen deficiency (below this, it starts to be deficient for human safety in many contexts). The other options: 23.5% is above normal, 15.5% and 10.5% are more severe deficiency but the key threshold for "deficient" (starting point) is 19.5%.

Answer:

The option with 19.5% (the second option, e.g., if labeled as B. 19.5% then B. 19.5%)