I have been living in the United States for 20 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：考查时态和倒装句。当否定词或者半否定词放在句首的时候，使用部分倒装的形式。本句的seldom（很少）放在句首，后面使用部分倒装的形式，根据前句中的现在完成时，说明本句也使用现在完成时。句意：我在美国生活了20年，但是很少像现在这样感觉到孤单。故A正确。