单项选择题
A．Construction of buildings is good for trees. B．Some trees' trouble comes from human beings. C．People should keep trees beautiful. D．Trees are not disturbed by changes around them.
A．that people should protect trees B．that people should plant more trees C．what trees can be used for D．what kinds of trees look beautiful
A．cutting down unhealthy trees B．making changes around the trees C．cooling and cleaning the air around the trees D．protecting the environment for trees growth