He tried to answer all the questions at once, but it was overwhelming.

at 是介词，表示"在...时候"或"处于某种状态"。 once 是副词，意思是"一次"或"曾经"。 at once 是一个固定短语，意思是"同时"或"立刻"，表示多个动作在同一时间发生。