So difficult         it to live in an English-speaking country that I determined to learn English well.

A．I have felt

B．have I felt

C．I did feel

D．did I feel

发生鼻源性颅内并发症最少见的鼻窦是（）。

A.上颌窦

B.额窦

C.筛窦

D.蝶窦

E.无差别