Across the United States, scientists are mounting what may become the most innovative agri

cultural research drive since the 1920s,when hybrid corn was developed. Surprisingly, the new genetic revolution is not taking place in America's fields. Instead, it is occurring in biology laboratories, for it involves the deliberate manipulation in test tubes of the genes of crop plants. This genetic engineering may prove the biggest boon to agriculture since plant breeding began.

The new concepts grew out of the bioengineering of bacteria for the production of such things as human hormones and vaccines for viral diseases. Plant cells, however, are far more complex than bacteria, and it will probably take many years for today's encouraging laboratory results to have a major impact on the farm. In fact the payoff may not come until the next century.

But although biotechnologists are still in the earliest phases of this new field of science, they are already actively exploring ways to redesign plants so they will use sunlight more efficiently, resist viruses and other pests, grow in hot or dry areas, in saline soils or in the presence of pesticides, and perhaps even make their own fertilizer out of nitrogen in the air. In addition, scientists have had early success in making wholly new plants that are unavailable by conventional plant breeding--a potato-tomato combination, for example.

The new technology holds the promise of virtually limitless horizons in food production. Only imagination sets the limits: frost-resistant wheat, tropical potatoes, saltwater rice, a plant producing a combination of a pea and a carrot--all may be with us one day.

What is the main topic of this passage？

A．Bacteria bioengineering.

B．Genetic engineering.

C．Agricultural technology.

D．Virus biotechnology.