正常胆囊壁的厚度为（）
A.>1mm
B.>2mm
C.>3mm
D.>4mm
E.>5mm
参考答案：C
医院感染研究的主要对象是
A．住院病人B．探视者C．医护人员D．门诊病人E．陪护家属
I _____ an exchange student here for a year now. [ ]
A. am
B. was
C. have been
D. had been
