Why did early civilizations begin in river valleys?

because food which is one of the major necessity for survival was met there

Explanation:

Most civilizations began near rivers and lakes. The people needed the water for drinking and growing plants, a skill learned during the Neolithic Revolution. River Valleys became ideal locations for cities and settlements because of the fertile land surrounding the river beds.

Explanation:

haploid cells;

hypoxemia refers to a below usual concentration of oxygen in the blood, mainly in the arteries. hypoxemia is an indication of an issue associated with circulation or breathing and may lead to various symptoms like shortness of breath.  

the condition is identified by determining the level of oxygen in the blood sample obtained from an artery. it can also be predicted by determining the oxygen saturation of the blood with the application of a pulse oximeter.  

the normal arterial oxygen is about 75 to 100 mm hg. the values underneath 60 mm hg is generally considered with the requirement for supplemental oxygen. the normal pulse oximeter readings generally range between 95 to 100 percent. the values under 90 percent are regarded as low.  

in the given case, the oxygen saturation is measured at 93 percent, hence, there is no need to administer oxygen, that is, one should withhold oxygen.