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  3. When a disease of epidemic proportions threatens the public, scientists immediately get to work, trying to locate the source of

When a disease of epidemic proportions threatens the public, scientists immediately get to work, trying to locate the source of

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问题     When a disease of epidemic proportions threatens the public, scientists immediately get to work, trying to locate the source of affliction and find ways to combat. Vaccination is one of the effective ways to protect the (1)_____ population of a region or country which may be (2)_____ grave risk. The process of vaccination allows the patient’s body to (3)_____ immunity to the virus or disease so that, if it is encountered, one can fight it (4)_____ naturally. To accomplish this, a small weak or dead (5)_____ of the disease is actually injected into the patient in a controlled environment, (6)_____ his body’s immune system can learn to fight the invader (7)_____. Information (8)_____ how to penetrate the disease’s defenses is (9)_____ 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 (10)_____ the patient later come into contact with the real problem, his body is well equipped and trained to (11)_____ with it, having already done so before.
    There are, however, dangers (12)_____ in the process. (13)_____, even the weakened version of the disease contained in the vaccine proves (14)_____ much for the body to handle, resulting in the immune system (15)_____, and, therefore, the patient’s death. Such is the case of the smallpox vaccine, (16)_____ to eradicate the smallpox epidemic that nearly (17)_____ the whole Native American population and killed massive numbers of settlers. (18)_____ 1 in 10,000 people who receive the vaccine (19)_____ the smallpox disease from the vaccine itself and dies from it. Consequently, the process, which is truly a (20)_____, may indeed hide some hidden curses.
Notes:
proportions(pl.)规模；程度；大小。affliction(疾病)痛苦。vaccination n.接种疫苗。eradicate v.根除，消灭。

选项 A、On occasion
B、By contrast
C、In addition
D、On the contrary
答案A
解析 逻辑搭配。on occasion(有时，不时)符合上下文的逻辑关系。by contrast(对照之下)，in addition(此外)，on the contrary(相反)均与上下文的逻辑关系相悖，故不能入选。
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考研英语一

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