The world's huge population has put unsustainable pressure on the earth's three natural systems—its fishing grounds, forests, and grasslands—on which we all depend for food. (2) The ocean is suffering greatly from the effects of overpopulation. (3) It is folly to view the ocean as an infinite source of food that simply needs better harvesting techniques. (4) The areas that are close to shore supply humans with virtually all their fish, but these areas are also the most polluted. (5) Modern fishing fleets equipped with echo sounders and sonar are devastating these fragile fish populations. (6) The second natural system, the forests, is also in danger. (7) Each year the world's forests shrink by an area the size of Cuba. (8) In Peru and Chile, for example, vast areas have been cut down for firewood and farming. (9) Incredibly, it takes about 850 acres of timber to produce just the Sunday edition of the New York Times. (10) Pressure on the world's forests will increase even further, as the demand for newsprint and packaging soars worldwide. (11) The third global life-support system, the grasslands, is also under mounting pressure. (12) Many U.S. farmers in the Midwest, for example, have bulldozed the rows of trees between their fields. (13) By reducing the force of the wind, these trees kept the topsoil from blowing away. (14) Now that many of the trees are removed, some fear a repeat of the dust storms that plagued farmers in the 1930s. (15) Back then, Oklahoma was the main victim; this time, much of the Midwest would be hit. D7.

The keys to the important ideas in this passage are
a. enumerations.
b. definitions.
c. an enumeration and definitions.