题目：
| In 1957 a doctor in Singapore noticed that hospitals were treating an unusual number of influenza-like cases. Influenza is sometimes called "flu" or a "bad cold". He took samples from the throats of patients in his hospital and was able to find the virus of this influenza. There are three main types of the influenza virus. The most important of these are type A and B, each of them having several subgroups. With the instruments at the hospital, the doctor recognized that the outbreak was due to a virus in group A, but he did not know the subgroup. Then he reported the outbreak to the World Health Organization in Geneva. WHO published the important news alongside reports of a similar outbreak in Hong Kong, where about 15-20 percent of the population had become ill. As soon as the London doctors received the package of throat samples, doctors began the standard tests. They found that by reproducing itself with very high speed, the virus had grown more than a million times within two days. Continuing their careful tests, the doctors checked the effect of drugs against all the known subgroups of virus type A. None of them have any protection. This, then, was something new, a new influenza virus, against which the people of the world had no help whatever. Having found the virus they were working with, the two doctors now dropped it into the noses of some specially selected animals, which get influenza much as human beings do. In a short time the usual signs of the disease appeared. These experiments proved that the new virus was easy to catch, but that it was not a killer. Scientists, like the general public, call it simply Asian flu. The first discovery of the virus, however, was made in China before the disease had appeared in other countries. Various reports showed that the influenza outbreak started in China, probably in February 1957. By the middle March it had spread all over China. The virus was found by Chinese doctors early in March. But China is not a member of the WHO and therefore does not report outbreaks of disease to it. Not until two months later, when travelers carded the virus into Hong Kong, from where it spread to Singapore, did the news of the outbreak reach the rest of the world. By this time it was well on its way around the world. |
Without counting the subgroups, there are ______type (s) of influenza.
A．one
B．two
C．three
D．four
答案：
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