he was appointed___ headteacher of the class为什么横线处不用填冠词?

he was appointed___ headteacher of the class为什么横线处不用填冠词?
能否讲得详细点 哪些具体职位或职务呢 Tom hasn't got an answer to the question of___it is necessary to change schools.A where Bif C whether D that
The students talked and laughed so loudly in the hall entrance that I had to shout (to be heard)怎么解释 Only then (did they realize)that the little boy had lost his parents 怎么解释？