The H1N1 flu spread very quickly at the beginning, , many people were ill.
A. therefore
B. instead
C. in advance
D. however
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(答案→)A. therefore
B. instead
C. in advance
D. however
解析：考查词义辨析。Therefore 因此，instead 代替，in advance 提前，however
然而，句意：H1N1病毒在开始传播的非常快，因此，许多人病了。所以选A。