In a science-fiction movie called "Species", a mysterious signal from outer space turns out to describe the genome of an unknown organism. When the inevitable mad scientist synthesizes the DNA described by the instructions, the creature he breeds from it turns out to resemble Natasha Henstridge, an athletic actress. Unfortunately, the alien harbors within her delicate form the destructive powers of a Panzer division, and it all ends badly for the rash geneticist and his laboratory.
Glen Evans, chief executive of Egea Biosciences in San Diego, California, acknowledges regretfully that despite seeking his expert opinion--in return for which he was presented with the poster of the striking Mr Henstridge that hangs on his office wall--the producers of "Species" did not hew very closely to his suggestions about the feasibility of their script ideas. Still, they had come to the right man. Dr Evans believes that his firm will soon be able to create, if not an alien succubus, at least a tiny biological machine made of artificial proteins that could mimic the behavior of a living cell.
Making such proteins will require the ability to synthesize long stretches of DNA. Existing technology for synthesizing DNA can manage to make genes that encode a few dozen amino acids, but this is too short to produce any interesting proteins. Egea’s technology, by contrast, would allow biologists to manufacture genes wholesale. The firm’s scientists can make genes long enough to encode 6,000 amino acids. They aim to synthesize a gene for 30,000 amino acids within two years.
Using a library of the roughly 1,500 possible "motifs" or folds that a protein can adopt, Egea’s scientists employ computers to design new proteins that are likely to have desirable shapes and properties. To synthesize the DNA that encodes these proteins, Egea uses a machine it has dubbed the "genewriter". Dr Evans likens this device to a word-processor for DNA, on which you can type in the sequence of letters defining a piece of DNA and get that molecule out.
As Egea extends the length of DNA it can synthesize, Dr Evans envisages encoding not just proteins, but entire biochemical pathways, which are teams of proteins that conduct metabolic processes. A collection of such molecules could conceivably function as a miniature machine that would operate in the body and attack disease, just as the body’s own defensive cells do. Perhaps Dr Evans and his colleagues ought to get in touch with their friends in Hollywood.
The passage is primarily （）in the development of the thesis.
A. persuasive
B. descriptive
C. narrative
D. expository
，x="_________" ,其中y = ___________。