阅读理解
Although it is mankind's inevitable fate to continually progress on the path of technology, there will always be the obstacle of resistance to overcome. Be it in the form of a protester who deplores implementation of the latest gadget or a dissenting scientist who disagrees with his colleagues' methods of research, it is an unavoidable fact that every step toward the future must be a hard-fought one. The latest battleground is one that has to do with the essence of nature itself: the bioengineering of certain plant species for human consumption. By learning about, isolating and finally manipulating the individual genes and strings of DNA that compose a plant species, geneticists are able to create new breeds of plants in the laboratory. These plants are able to live longer in harsher environments, provide better nutrition, and sustain themselves with a minimum of human care. By changing the composition of the plants at a cellular level and chemically combining elements of different plants that don't normally cross-pollinate in nature, science is ushering in a wave of new food items that earth has never seen before. Many argue that they are achieving the best of both worlds, the best that each individual species has to offer, conveniently contained within one product. Some scientists even go so far as to argue that these foods are better for humans than anything nature itself has ever created. The opposition has its own theories, however. Due to the relatively new nature of these experiments, it is questionable whether these items are truly safe for human consumption, despite government approval. Taking a broader view, they contend that by combining species that normally do not go together, we may be achieving the WORST of both worlds, meaning that diseases that are normally contained within one species will combine as well. Thus, they may be made stronger by mutation, possibly endangering earth's agricultural food supply if unleashed upon helpless natural species that have never had to deal with such enemies before. The possibilities that come with such a warning are frightening. A bio-engineered disease could wreak havoc on the delicate balance created by nature, possibly decimating the plant population of earth and depriving mankind of its food source. Clearly, the words of those opposed to progress must be considered on this issue, for the stakes are simply too high to be taken without an extreme degree of caution.
单选题21.The word "cross-pollinate"(line 5, para. 2)in the context probably means "______".