Ellen was a painter of birds and of nature, , for some reason, had withdrawn from all huma

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A. which
B. who
C. where
D. whom
解析：who引导非限定性定语从句，在从句中做主语，修饰先行词Ellen（人名）。Which修行先行词是物的定语从句；whom修饰先行词为人，且在从句中做宾语；where修饰先行词是地点名词，且在从句中地点状语，均不符合语境，故排除。