When you hold your breath, which of the following gas changes in blood would first lead to the urge to breathe?

Approximately seventy percent of carbon-dioxide absorbed by the blood will be transported to the lungs

The figure shows a diagrammatic view of human respiratory system with labels A, B, C and D. Select the option which gives correct identification and main function and / or characteristic.

People who have migrated from the planes to an area adjoining Rohtang Pass about six months back