I have been living in the United States for twenty years but seldom so lonely as now.A.

问题补充：

I have been living in the United States for twenty years ,but seldom so lonely as now.

A. have I felt

B. I had felt

C. I have felt

D. had I felt

答案：

(答案→)A. have I felt

B. I had felt

C. I have felt

D. had I felt

解析：句意：我在美国已经住了了，但很少像现在这样孤独。由seldom 可知需要使用部分倒装结构，同时我们不难判断应使用现在时态。