The problem with self-selection in research is that a. it violates ethical guidelines. b. it makes causal interpretations difficult. c. people often overestimate their own contributions to the research. d. people are often unaware which condition they should choose to be in.

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.