When deeply absorbed in work, ______ he often was, he would forget all about eating or sleeping.
A.that
B.which
C.where
D.when
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B解析考察定语从句（非限定性）。本题主句部分完整，且选项包含that, which因此在考查定语从句。应先分析空格之后的句子成分，本题缺少was的表语，且空格前有逗号，应选which。句意：当他集中精力工作时，这种情况常有发生，他总是废寝忘食。