Before Constantine even started to design a man-made defense, what do you see that is a natural barrier and defense? A) Bodies of Water.
B) Mountain Ranges.
C) Walls.
D) Peninsula.

the answer is migration

the underground railroad (apocope of the metropolitan railroad) is a system of urban trains located within a city and its metropolitan area. it is characterized for being a massive transport of passengers in the big cities, uniting diverse zones and their environs, with high capacity and frequency and in segregated form to other systems of transport. metropolitan railroads are built underground or on the surface, although most systems use mixed models where sections are combined in both modalities.

1 - as a result, the metro tends to reduce car travel for two reasons: first, due to the greater transfer of users from the car and, second, due to the reduction in the acquisition of cars in homes located in sectors near the stations of the metro.

2 -the metro allows to contribute effectively to the achievement of all the objectives of an urban development policy: it improves the efficiency of the city's economy by reducing travel costs and also generates a higher level of activities in the city center, taking advantage of of the agglomeration economies.

3 -it is a clean means of transport, since it uses renewable energy (usually electricity), taking care of the environment and avoiding excessive pollution.

before there were tanks, machine guns, windmills, and solar panels, the first civilizations emerged that create the history of humanity which provided the larger context for western civilization. uses of weaponry and technology were used very differently in the ancient civilizations than they are today. relating back to the times of the ancient mesopotamian civilization, technology kept advancing throughout the ancient egyptian civilization into the ancient greek civilization, and then finally the ancient roman civilization. as techniques and strategies were past down from civilization to civilization, each shared its own similarities, as well as varied differences. one trait they had in common was to commit violence and economically overpower one another. technology has ultimately advanced due to the knowledge transferred from generation to generation. uses of sophisticated weaponry and devices have created more opportunity for warfare and improvement in the work place. ancient civilizations became more powerful due to the advancements of technology. technology did not only create the use of violence, but it also the economies grow and become stronger, because production of trading goods rose like never before. the rivers, such as the nile, tigris, and euphrates all flourished, as well as destructed civilizations. each civilization had different uses for these rivers, however, some benefited more than others. when analyzing ancient civilizations, there   are things you must understand

explanation: