钩端螺旋体多价疫苗接种后其免疫力可维持()
A.有效1~2年
B.有效1年以内
C.终生免疫
D.有效2~5年
E.有效5~10年
参考答案:B
— Have you been here before?
— Yes, it's the second time ______ I ______ the Great Wall. [ ]
A. when; visited
B. that; visited
C. when; have visited
D. that; have visited
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