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Sugar cane was once a wild grass that grew in New Guinea and was used by local people for roofing their houses and fencing their gardens.Gradually a different variety evolved which contained sucrose (蔗糖) and was chewed on for its sweet taste.Over time, sugar cane became a highly valuable commercial plant, grown throughout the world.(46)

Sugar became a vital ingredient in all kinds of things, from confectionery(糖果点心) to medicine, and, as the demand for sugar grew, the industry became larger de more profitable.(47) Many crops withered(枯萎) and died, despite growers attempts to save them and there were fears that the health of the plant would continues to deteriorate.

In the 1960s, scientists working in Barbados looked for ways to make the commercial species stronger and more able to resist disease.They experimented with breeding programmes, mixing genes from the wild species of sugar cane, which tends to be tougher, with genes from the more delicate, commercial type.(48) This sugar cane is not yet ready to be sold commercially, but when this happens, it is expected to be incredibly profitable for the industry.

(49) Brazil, which produces one quarter of the world's sugar, has coordinated an international project under Professor Paulo Arrudo of the Universidade Estaudual de Campinas in Sao Paulo.Teams of experts have worked with him to discover more about which parts of the genetic structure of the plant are important for the production of sugar and its overall health.

Despite all the research, however, we still do not fully understand how the genes in sugar cane.(50) This gene is particularly exciting because it makes the plant resistant to rust, a disease which probably originated in India, but is now capable of infecting sugar cane across the world.Scientist believe they will eventually be able to grow a plant which cannot be destroyed by rust.

a.the majority of the world's sugar now comes from this particular commercial species.

b.unfortunately, however, the plant started to become weaker and more prone to disease.

c.eventually, a commercial plant was developed which was 5 percent sweeter than before, but also much stronger and less likely to die from disease.

d.since the 1960s, scientists have been analyzing the mysteries of the sugar cane's genetic code.

e.one major gene has been identified by dr angelique d' hont and her team in montpelier, france.

f.sugar cane is now much more vigorous and the supply of sugar is therefore more guaranteed.

参考答案：46a.47b.48c.49f.50e.