The frontiers when taken into consideration, maps which establishes the European frontiers were seen to appear from the 18th century only and not prior to that.
<u>Explanation:
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The ideology of formation of state or state boundary were emerged with the period of 17th century and even mid of 16th century witnessed it. The establishment of frontier though started with 18th century only.
The different topographical features were considered to describe the fronts such as the rivers, mountains. And, even the landmarks such as forts or towers were considered as the border distinction.
Answer: if I'm reading it right, because it's a bit blurry, a is 900 b is 900 c is 1500 and d is 1000
