【单选题】The work to stamp out smallpox was made more difficult by______.
A.people's unwillingness to report cases
B.the lack of vaccine
C.the backwardness of the region
D.the incessant local wars
【单选题】The work to stamp out smallpox was made more difficult by______.
A.people's unwillingness to report cases
B.the lack of vaccine
C.the backwardness of the region
D.the incessant local wars
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