A:be careful
B:to be careful
C:to care
D:to have been careful.
解:∵OE⊥AB,
∴∠EOB=____,
∴∠EOD+____=____,
又∵∠EOD=2∠BOD,
∴∠BOD=____,∠EOD=____,
∵OF⊥CD,
∴∠FOD=____,
∴∠EOF=____﹣____ =____.
生物圈包括____的底部____的大部和____的表面.