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

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：考查倒装句。否定词放在句首，使用部分倒装，题中的否定词为seldom，根据时态应用完成时态，所以助动词have 提前，选A。