Special trees that grow faster, fight pollution, produce better wood, and even sense chemical attacks are being planted by scientists in the US. When 40 per cent of Hawaii's US$14 million-a-year papaya  industry was destroyed by a virus five years ago, work began on creating genetically engineered trees. Researchers successfully introduced seeds that were designed to resist the virus.Since then, more and more people have been testing genetically engineered trees.Some researchers put special bacteria into trees to help them grow faster and produce better wood.Others are trying to create trees that can clean polluted soil.Meanwhile fruit farmers are looking for trees that are strong enough to resist worms, and paper companies want trees that produce more wood and therefore more paper. The Pentagon   even gave the researchers US$500,000 this year after they developed a pine tree that changes its colours if it senses a chemical attack.So far, the poplar, eucalyptus  , apple and coffee trees are among those being engineered.All this can be done today because we have a better understanding of tree genomes  . However, some people fear that the genetically engineered trees will cause dangerous results.They are worried that the new trees will breed with natural species and change the balance of the forest environment. "It could be destructive," said Jim Diamond, an environmentalist. "Trees are what is left of our natural environment and home to many endangered species." But researchers insist that science could give nature a fighting chance against both natural and man-made dangers.They hope to answer the critics by stopping the new trees from breeding, so their effect on the environment can be controlled. Which of the following was probably the first kind of trees being engineered?
Papaya. Pine. Apple. Poplar.