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
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(答案→)A. have I felt
B. I had felt
C. I have felt
D. had I felt
解析：本题考查倒装结构。句中否定词seldom放在句首，故用倒装结构，根据时态用现在完成时。句意：我在美国生活了20年了，但我很少像现在一样孤独。