sentences: an example of managing / maintaining the number / the use of something is replacing that something & resources must be managed ->  an example of managing the number of a resource is replacing that resource  trees are a kind of resource & a forest contains lots of plants / trees ->  trees are a kind of resource in a forest  an example of managing the number of a resource is replacing that resource & trees are a kind of resource in a forest ->  an example of managing the number of trees in a forest is replacing those trees  an example of managing the number of trees in a forest is replacing those trees & a tree can be replaced by planting a new tree each time one is cut down ->  an example of managing the number of trees in a forest is replacing trees by planting a new tree each time one is cut down  an example of managing the number of trees in a forest is replacing trees by planting a new tree each time one is cut down & timber companies cut down trees -> timber companies can manage the number of trees in a forest by planting trees to replace what they cut down