梅毒的潜伏期为 ()
A.<2周
B.2~4周
C.5~6周
D.7~8周
E.>8周
参考答案:B
解析:梅毒的传染源是梅毒病人,梅毒的潜伏期约为2~4周。
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If a computer crashes, you will lose the file you _________unless you save it regularly.
A.are working
B.work
C.will work
D.worked
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