Why do you get tired when you run? ?

Because the more someone exerts themselves the more oxygen and carbon dioxide out put ratio is greater. The more you run and the faster you run the more oxygen you loose which is supplied to your muscles including your lungs. 

After a while our leg muscles do not get enough oxygen.

Caffiene and drowezyness medication

I’m going to guess these are the choices: a) recreational skiing developed in the mid-19th century b) it was originally invented as a way of getting from place to place in snowy regions c) downhill skiing is currently the only common form of the sport d) it mostly evolved from snowshoeing- if that’s the cases then the answer would be c) downhill skiing is currently the only common form of the sport