According to the passage, only ______ trees can give us shade and products.

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．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