Explain why dosea mountain climber needs an oxygen supply at very high altitude even through the air still contains 21% oxygen

Someone pls ill give brainliest what would be the answer to this question question- miro mis libros y estudió la lección en silencio aquí (here). my answer- librería. my teacher said- try this one again! 'librería' is a false cognate; is doesn't mean 'library'.

En colombia los hombres de negocios

How many square miles of territory did paraguay lose in the war