On which of these landforms is the highest concentration of the russian population found?

The highest Russian population is found on the West Siberian Plain. The West Siberian Plain is approximately  6 kilometres from west to east and  670 kilometres from north to south. Half of the plain’s territory is elevated below 200 meters.

hydrogen is a compound or a single element and a element is a pure substence that cannot be separated into simpler subtances

clorine is a compound its a chemical substance that forms when two or more different elements combine

so both hydrogen and clorine are compounds