One reason the roman empire was difficult to conquer was because

a)the geographic features that surrounded it.
b)there were no people who considered rome an enemy.
c)there was not an economic reason to want to conquer it.
d)the lack of slavery there made it welcoming for all peoples.

There are two sources that serve as the foundation for conducting research on learning. the first source addresses characteristics of knowledge itself and the different ways in which we learn things. the second source focuses on what goes on in our minds and how that is theoretically represented. references: