The system is the most common system of measurement used in the world today

The metric system

Explanation:

two most common systems of measurement: the metric system, used widely in Europe and most of the rest of the world, and the Imperial or British system, a form of which is now chiefly used in the USA.

b. the carbon cycle ensures living things have a source of hydrogen

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.