单项选择题
A.Branches. B.Grasses. C.Mud. D.Straw.
A.It affects the growth of trees. B.It has been increasing since the Ice Age. C.It is determined by the chemicals in the air. D.It follows a certain cycle.
A.The lifecycle of trees. B.The number of trees. C.The intensity of solar burning. D.The quality of air.