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

(A) their number has declined compared with

(B) their numbers have declined compared with

(C) their numbers have declined since

(D) their numbers have declined in comparison to those of

(E) the number of them declined since