A person is hiking on a mountain. His breathing rate is increasing while he is walking up hill. The reason is ………
1) oxygen levels are generally less as we move uphill
2) our muscles have to use more oxygen while walking up
3) oxygen levels are more on mountains
4) our muscles have to use less oxygen
A) 1 only
B) 1 and 2
C) 1 and 4
D) 4 only