语法与词汇 29.The question of______the traffic accident happened hasn't been answered yet, though the police have made on-the-spot investigation.
【正确答案】 C
【答案解析】of是介词。介词后面若加宾语从句，必须是特殊疑问词开头的宾从。从意思上来讲，是“如何(how)发生”，而不是“是否(whether)发生”。