In the United States proven oil reserves -- the amount of oil considered extractable from known fields -- are at the same level as they were ten years ago. Yet over this same period no new oil fields of any consequence have been discovered, and the annual consumption of domestically produced oil has increased. Which one of the following, if true, best reconciles the discrepancy described above? ['Oil exploration in the United States has slowed due to increased concern over the environmental impact of such exploration.', 'Conservation measures have lowered the rate of growth of domestic oil consumption from what it was a decade ago.', 'Over the past decade the annual consumption of imported oil has increased more rapidly than that of domestic oil in the United States.', 'Due to technological advances over the last decade, much oil previously considered unextractable is now considered extractable.']