What physical feature stretching between two bodies of water seems to be a northern border for Muslim lands

Which options most accurately assess the impact of transportation improvements (e.g., railroads, canals, steamboats, transcontinental railroad) on the settlement of the west? ***(select all that apply.)*** a. they led to an increase in population density. b. they made it easier to travel long distances. c. they in establishing towns. d. they led to the development of the fur trade. e. they allowed for international travel.