The answer is A. Maps establishing European frontiers only began to appear in the 18th century. With the emergence of territorial states in the sixteenth and seventeenth centuries borders began to be discussed in the context of state ideology. Different topographical features, not only rivers, but mountains and manmade landmarks such as fortresses, etc. began to serve as borders.
In the sixteenth century there was a noticeable change in connection with the depiction of political borders on maps. But with the improvement of measurement techniques in the eighteenth century, the concept of "border" acquired linear form. Linearity expressed frontiers.
