求助大全上的一道阅读题！

Although geneticmutations in bacteria and viruses can lead to epidemics, some epidemics arecaused by bacteria and viruses that have undergone no significant geneticchange. In analyzing the latter, scientists have discovered the importance ofsocial and ecological factors to epidemics. Poliomyelitis (poliomyelitis: n.小儿麻痹症, 急性骨髓灰白质炎), for example, emerged asan epidemic in the United States in the twentieth century; by then (by then: 到那时候), modernsanitation was able to delay exposure to polio (POLIOMYELITIS) untiladolescence or adulthood, at which time polio infection produced paralysis.Previously, infection had occurred during infancy, when it typically providedlifelong immunity without paralysis. Thus, the hygiene that helped prevent typhoidepidemics indirectly fostered a paralytic polio epidemic. Another example is Lymedisease, which is caused by bacteria that are transmitted by deer ticks. Itoccurred only sporadically during the late nineteenth century but has recentlybecome prevalent in parts of the United States, largely due to an increase inthe deer population that occurred simultaneously with the growth of the suburbsand increased outdoor recreational activities in the deer’s habitat. Similarly,an outbreak of dengue hemorrhagic fever became an epidemic in Asia in the1950’s because of ecological changes that caused Aedes aegypti, the mosquito that transmits the dengue virus, toproliferate. The stage is now set in the United States for a dengue epidemicbecause of the inadvertent introduction and wide dissemination of anothermosquito, Aedes albopictus.

(A) The deerpopulation was smaller in the late nineteenth century than in the mid-twentiethcentury.

(B) Interest inoutdoor recreation began to grow in the late nineteenth century.

(C) In recentyears the suburbs have stopped growing.

(D) Outdoorrecreation enthusiasts routinely take measures to protect themselves againstLyme disease.（A）

(E) Scientistshave not yet developed a vaccine that can prevent Lyme disease.