Seeing a student master a difficult concept is an example of an reward.

It is an example of extrinsic reward as this is a reward that is acquired after having to perform a task that has been difficult or has been a part of something big in accomplishing goals either in work or in school. It is considered to be an extrinsic reward because a teacher, having to see his or her student master a concept that is difficult, is already rewarding for them—which is referred to as an extrinsic reward.

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

the first one is d and the second a