The possibility pleasant smells might reduce pain has recently been suggested by new rese

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A. where
B. what
C. which
(答案→)D. that
解析：考查同位语从句，同位词是The possibility，从句是对The possibility内容的说明，用that连接同位语从句，that只起到连接的作用，不做成分。句意：最近的研究说明愉快的味道可以减少疼痛的可能性。选D。