A) what two processes account for adding free oxygen to the precambrian atmosphere? b) what do we mean by free oxygen, and c) which of these two processes were most important?

Historians are researchers of the past or people who understand the history of people. with this field, they must possess qualities such as being unbiased, patient, efficient reader, has a knack for logic and critical thinking and reasoning, disciplined, open-minded, resourceful, with good analytical ability, good reasoning skills and problem solving!