请选择DCBA+A'\)G7]2R7a9S8Y9R8^.~Every year millions of trees ___________to prevent floods, droughts and sandstorms.(E2O(z+R/x-\5r2P4n
A. plant(E2O(z+R/x-\5r2P4n
B. are planted(E2O(z+R/x-\5r2P4n
C. are planting(E2O(z+R/x-\5r2P4n
D. were planted(E2O(z+R/x-\5r2P4n