A、It provides us food and place to live. B、It contacts with us from birth to death. C、It works with genes to shape who we are. D

M: Hey, Cathy. You didn’t say even a word in the last ten minutes. What are you doing on earth? W: Well, everybody is talking about genes nowadays, but do genes determine everything about us? M: (19) Genes and the environment work together to shape who we are. As the environment includes everything we experience and comes into contact with us from birth to death, it plays an extremely complex role.
W: What is gene, then?
M: Even scientists disagree on how to define a gene exactly. (20)Generally, a gene is a sequence of DNA that tells us what a single protein does. Proteins do the work of the body, building structures and stimulating biochemical reactions.
W: How genetically similar are we to one another?
M: Individual humans are extremely similar to each other. They share 99. 9% of their DNA, no matter which race they belong to.
W: How genetically similar are we to other species?
M: Scientists have been amazed at how genetically similar humans are to other species.  Humans share between 79% and 80% of their genes with mice, and about 99% of their DNA with chimpanzees. That means that chimpanzees are more closely related to humans than to other animals, such as monkeys, whales, or cats.
W: (21)Will this lead to cure for cancer and other serious diseases?
M: (21)Of course! Virtually, every disease has some genetic cause.
19. How does environment work to human?
20. What is a gene according to the man?
21. What can be inferred about genes from the conversation?