某小儿在幼儿园接触过处于卡他期的百日咳患儿,此时应对该健康小儿观察()
A.28天
B.21天
C.14天
D.7天
E.3天
参考答案:B
Lily is _______ English girl. ________ is a student.
A. a, He B. an, She C. an, He
玉米一般千粒重为()g.
A.50-100
B.100-200
C.200-250
D.250-350