He was very tired, and he fell asleep. [ ]
A. therefore
B. so
C. but
答案：A
外部过电压分为（）
A、直接雷电过电压
B、雷电感应过电压
C、操作过电压
D、谐振过电压
知识库是数据库与（）这两种技术的结合。
