Ravi,who lived at sea level,had around $5$ million $RBC$ per cubic millimeter of his blood. Later when he lived at an altitude of $18,000 \ ft$,he showed around $8$ million $RBC$ per cubic millimeter of blood. This is an adaptation because

• A
  At high altitude he ate more nutritive food
• B
  He had pollution-free air to breathe
• C
  At high altitude $O_2$ level is less,hence more $RBCs$ were required to absorb enough oxygen
• D
  At high altitude there is more $UV$ radiation which enhances $RBCs$ production