When Julius Caesar made his triumphal entrance into Rome in 45 BC, he celebrated by giving

a feast at which thousands of guests ate gorged (狼吞虎咽) poultry, seafood and game. Similar celebrations featuring excessive consumption of animal flesh have marked human victories-in war, sport, politics and commerce-since our species learned to control fire. Throughout the developing world today, one of the first things people do as they climb out of poverty is to shift from their peasant diet of mainly grains and beans to one that is rich in pork or beef. Since 1950, per capita consumption of meat around the globe has more doubled.

Meat, it seems, is not just food but reward as well. But in the coming century, that will change. Much as we have awakened to the full economic and social costs of cigarettes, we will find we can no longer subsidize (补助) or ignore the costs of mass-producing cattle, poultry,, pigs, sheep and fish to feed our growing population. These costs include hugely inefficient use of fresh water and land, heavy pollution from livestock feces, rising rates of heart disease and other degenerative illness, and spreading destruction of the forests on which much of our planet's life depends.

First, consider the impact on supplies of fresh water. To produce l kg of feedlot beef requires 7kg of feed grain, which takes 1 000kg of water to grow. Pass up one hamburger, and you'll save as much as water as you save by taking 40 showers with a low-now nozzle (喷嘴). Yet in the U. S. , 70% of all the wheat, corn and other grain produced go to feeding herds of livestock. Around the world, as more water is diverted to raising pigs and chickens instead of producing crops' for direct consumption, millions of wells are going dry, India, China, North Africa and the U. S. are all running freshwater deficits, pumping more from their .aquifers (蓄水层) than rain can replenish. As populations in water scarce regions continue to expand, governments will inevitably act to cut these deficits by shining water to grow food, not feed. The new policies will raise the price of meat to levels unaffordable for any but the rich.

That prospect will doubtlessly provoke protests that direct consumption of grain can't provide the same protein that meat provides. Indeed, it can't. But nutritionists will attest that most people in the richest countries don't need nearly as much protein as we are currently getting from meat, and there are plenty of vegetable sources--including the grains now wasted on feed--that can provide the protein we need.

In the opening paragraph, the author introduces his topic by ______.

A．justifying an assumption

B．giving an example

C．making a comparison

D．explaining a phenomenon