________ I came into the office, the teachers were having a meeting．

________ I came into the office, the teachers were having a meeting．

A.While

B.When

C.Once

D.Since

答案及解析： B

解析：

while引导的从句中动词常常是表示延续的动词或表示状态的连系词be; when表示“当……的时候”；

once表示条件；

since表示时间时意思是“自从……”。根据句意可知选B项。