A is any known starting point.

Baseline

Explanation:

Baseline is a starting point or the onset of a thing.

Baseline line is always very important and could be a basis of comparison for something.

Baseline is used in many discipline this include medicine which is the very information found at the beginning of a study.

For every individual the base line or start up is also very vital and crucial and its often taken with all seriousness to avoid error that may not be easily corrected.

answer; i believe that the correct answer is ( rivers);