Question ID:PT44 S1 Q20 Passage:The survival of nerve cells, as well as their performance of some specialized functions, is regulated by chemicals known as neurotrophic factors, which are produced in the bodies of animals, including humans. Rita Levi-Montalcini's discovery in the 1950s of the first of these agents, a hormonelike substance now known as NGF, was a crucial development in the history of biochemistry, which led to Levi-Montalcini sharing the Nobel Prize for medicine in 1986.In the mid-1940s, Levi-Montalcini had begun by hypothesizing that many of the immature nerve cells produced in the development of an organism are normally programmed to die. In order to confirm this theory, she conducted research that in 1949 found that, when embryos are in the process of forming their nervous systems, they produce many more nerve cells than are finally required, the number that survives eventually adjusting itself to the volume of tissue to be supplied with nerves. A further phase of the experimentation, which led to Levi-Montalcini's identification of the substance that controls this process, began with her observation that the development of nerves in chick embryos could be stimulated by implanting a certain variety of mouse tumor in the embryos. She theorized that a chemical produced by the tumors was responsible for the observed nerve growth. To investigate this hypothesis, she used the then new technique of tissue culture, by which specific types of body cells can be made to grow outside the organism from which they are derived. Within twenty-four hours, her tissue cultures of chick embryo extracts developed dense halos of nerve tissue near the places in the culture where she had added the mouse tumor. Further research identified a specific substance contributed by the mouse tumors that was responsible for the effects Levi-Montalcini had observed: a protein that she named "nerve growth factor" (NGF).NGF was the first of many cell-growth factors to be found in the bodies of animals. Through Levi-Montalcini's work and other subsequent research, it has been determined that this substance is present in many tissues and biological fluids, and that it is especially concentrated in some organs. In developing organisms, nerve cells apparently receive this growth factor locally from the cells of muscles or other organs to which they will form connections for transmission of nerve impulses, and sometimes from supporting cells intermingled with the nerve tissue. NGF seems to play two roles, serving initially to direct the developing nerve processes toward the correct, specific "target" cells with which they must connect, and later being necessary for the continued survival of those nerve cells. During some periods of their development, the types of nerve cells that are affected by NGF‚ primarily cells outside the brain and spinal cord‚ die if the factor is not present or if they encounter anti-NGF antibodies. Stem:Which one of the following is most strongly supported by the information in the passage? Correct Answer Choice:DChoice A:Some of the effects that the author describes as occurring in Levi-Montalcini's culture of chick embryo extract were due to neurotrophic factors other than NGF. Choice B:Although NGF was the first neurotrophic factor to be identified, some other such factors are now more thoroughly understood. Choice C:In her research in the 1940s and 1950s, Levi-Montalcini identified other neurotrophic factors in addition to NGF. Choice D:Some neurotrophic factors other than NGF perform functions that are not specifically identified in the passage. Choice E:The effects of NGF that Levi-Montalcini noted in her chick embryo experiment are also caused by other neurotrophic factors not discussed in the passage.