What is the most common acid and base?
and why?

Acids and bases are used in most many chemical reactions in chemistry . They are responsible for most colour changes in a chemical reaction and are used to adjust the pH of chemical solutions.

Explanation:

in cartography, a contour line (often just called a "contour") joins points of equal elevation (height) above a given level, such as mean sea level. a contour map is a map illustrated with contour lines, for example a topographic map, which thus shows valleys and hills, and the steepness or gentleness of slopes.

explanation:

fe 55.85 g/mol and h2o 18.02 g/mol.