单选题 A new biotechnology procedure that could become commercially available in as little as two to four years is "transgenosis", which permits scientists to create an animal with specific traits by adding, removing, inactivating, or repairing genes in an embryo. The additional genes can come from any source. For example, if a gene of interest occurs in mosquitoes—say, one that codes for resistance to a certain disease—it can be removed and placed in the embryo of a farm animal, the several strains of commercially useful transgenic farm animals that will probably emerge in the next few years could include leaner pigs, poultry resisting to influenza or other deadly diseases, sheep with wool that is easier to wash, and goats that produce valuable pharmaceuticals in their milk. The simplest way to make transgenic animals is to inject a gene into a one-cell embryo and then implant the embryo in another animal. Under the right conditions, the new gene joins one of the embryo's strands of genes. Each cell created as the embryo divides gets a copy of the new gene. An alternative technique is to incorporate the gene into a type of virus known as a retrovirus that has been modified so it cannot reproduce itself after entering a cell. The virus, which cannot cause disease, delivers the gene to the cell's nucleus. Often this method is better than gene injection because a retrovirus always delivers just one gene, and the gene is always undamaged and complete.

单选题 The new biotechnology procedure discussed in this passage mainly concerns______.

单选题 In the first sentence, "to become commercially available" means______.

单选题 The transgenic farm animals mentioned in this passage are______.

单选题 Which of the following is NOT true?

单选题 The tone of this passage is______.