I think it ________ question ______.

正确答案为C.
用too、so、as等词在修饰带形容词的单数名词时,要用
“too/so/as + 形容词 + a/an + 单数名词”
这样的结构.
例如：
He's so interesting a man that we all like him.
It's too hard a job for me to do.
It's as difficult a problem as the one I did yesterday.
such后的结构则是：
such + a/an + 形容词 + 单数名词
例如：He's such an interesting man.
题目的第二项考的是too ...to ...结构,too不宜改成so或such.

B