Native Americans were able to prevent the encroachment of trees and maintain the tallgrass prairies by

A) establishing an extensive irrigation system to increase soil moisture, thus favoring grasses.
B) plowing much of the land for agriculture.
C) restricting the migrations of bison, which caused the bison to browse heavily on tree seedlings and thus kill them.
D) setting fires periodically to kill the trees.
E) clear- cutting the forests for timber and fuel.