when you look at the cord coming out of the lamp on your desk, you will notice that there is a protective coating that surrounds and encapsulates the electricity-conducting wire inside. if you were to relate that model to the human neuron, the protective coating would refer to the

Myelin

Explanation:

Myelin is a fatty substance that surrounds nerve cell axons to insulate them and increase the rate at which information is passed along. The myelin can be compared to the protective coating of an electrical wire.

first, the romans stayed isolated because they had to defend their own territories from neighboring cities that were harassing them, like the etruscans. second, once the romans managed to defeat their neighbors, and started to expand their territory, they started to build roads and bridges. finally, now that the romans have covered their territory with roads functioning as highways, they could mobilize their legions faster to assemble larger military units in a specific place.

i am pretty sure it is d.