题目选项分析

Although beavers can still be found throughout the western United States, their numbers have declined in comparison to those of the eighteenth century, when the abundance of beavers lured armies of European trappers into the Rocky Mountain region to slaughter them for their pelts.

Although beavers can still be found throughout the western United States, their numbers have declined since the eighteenth century, when the abundance of beavers lured armies of European trappers into the Rocky Mountain region to slaughter them for their pelts.