What are the causes of a low blood oxygen level?

Which action is characterized of mental illness 1.brushing teeth every hour because ones mouth feels dirty ? 2. forgetting to eat breakfast on a busy day? 3.crying three days over a relationship breakup? 4.feeling embarrassed about going to school after failing in front of friends?

Ireally need to know this answer asap

Developing a positive identity to increase your level of self esteem true or false