伤寒较为常见的严重并发症是（）
A.肠出血
B.肠穿孔
C.中毒性肝炎
D.中毒性心肌炎
E.支气管肺炎
参考答案：A
— Well, ________ I have to fetch my son in the kindergarten.
— Of course. Thank you very much for helping us. [ ]
A. may I excuse myself now?
B. let me go first.
C. I'm sorry.
D. see you later.
显微镜的光圈有何功用？（）
A.调整焦点
B.接收光线，并反射
C.调节光亮
D.支持镜台
