Question

On a map, black and red numbers most likely indicate the __________ .

Answer 1

The answer is: Man made infrastructure.

This is more relevant in a 'political' map which shows the political boundaries and representations of countries.

More precisely,

Black color shows railway lines, roads, bridges, borders, and names of places
Red color indicates Major Highways, urban areas, airports, military installations etc.

These lines/markings/colors might not be seen physical maps, which feature the geography of the land.
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