What is the function of the endocrine system?

Endocrine glands release hormones into the bloodstream. This lets the hormones travel to cells in other parts of the body. The endocrine hormones help control mood, growth and development, the way our organs work, metabolism , and reproduction.

Explanation:

B. When oxygen levels in the blood decrease, the respiratory center sends signals to the diaphragm to increase the rate at which it contracts and relaxes.

Explanation:

According to the given situation, as it is already mentioned that Robert runs before going home from school. This physical activity is likely to increase the breathing rate. The increase in breathing rate is likely to increase the demand of oxygen in the body. With the decrease in concentration of the oxygen in the blood the respiratory system in the body will get activated. The respiratory system will send signals to the diaphragm below the lungs to expand and contract at a faster rate so as to admit more amount of oxygenated air and distribute it at a faster rate.