Q 1 :    

When you hold your breath, which of the following gas changes in blood would first lead to the urge to breathe?           [2015]

• Falling ${\mathrm{CO}}_2$ concentration

   

• Rising ${\mathrm{CO}}_2$ and falling ${\mathrm{O}}_2$ concentration

   

• Falling ${\mathrm{O}}_2$ concentration

   

• Rising ${\mathrm{CO}}_2$ concentration