The purpose of the author in writing this passage

单项选择题

When a disease of epidemic proportions rips into the populace, scientists immediately get to work, trying to locate the source of the affliction and find ways to combat it. Oftentimes, success is achieved, as medical science is able to isolate the parasite, germ or cell that causes the problem and finds ways to effectively kill or contain it. In the most serious of cases, in which the entire population of a region or country may be at grave risk, it is deemed necessary to protect the entire population through vaccination, so as to safeguard lives and ensure that the disease will not spread. The process of vaccination allows the patient"s body to develop immunity to the virus or disease so that, if it is encountered, one can ward it off naturally. To accomplish this, a small weak or dead strain of the disease is actually injected into the patient in a controlled environment, so that his body"s immune system can learn to fight the invader properly. Information on how to penetrate the disease"s defenses is transmitted to all elements of the patient"s immune system in a process that occurs naturally, in which genetic information is passed from cell to cell. This makes sure that, should the patient later come into contact with the real problem, his body is well equipped and trained to deal with it, having already done so before. There are dangers inherent in the process, however. On occasion, even the weakened version of the disease contained in the vaccine proves too much for the body to handle, resulting in the immune system succumbing, and, therefore, the patient"s death. Such is the case of the smallpox vaccine, designed to eradicate the smallpox epidemic that nearly wiped out the entire Native American population and killed massive numbers of settlers. Approximately 1 in 10,000 people who receives the vaccine contract the smallpox disease from the vaccine itself and dies from it. Thus, if the entire population of the United States were to receive the Smallpox Vaccine today, 3000 Americans would be left dead. Fortunately, the smallpox virus was considered eradicated in the early 1970"s, ending the mandatory vaccination of all babies in America. In the event of a reintroduction of the disease, however, mandatory vaccinations may resume, resulting in more unexpected deaths from vaccination. The process, which is truly a mixed blessing, may indeed hide some hidden curses.

The purpose of the author in writing this passage is ______.

A．to comment and criticize.
B．to demonstrate and argue.
C．to interest and entertain.
D．to explain and inform.