Researchers have found clear and convincing evidence happiness is the key to better healt

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A. where
B. whether
C. how
(答案→)D. that
解析：考查同位语从句。从句部分“happiness is the key to better health and longer lives”是主系表结构，不缺少成分，evidence证据、fact事实、question问题、doubt疑问、idea想法等词，后面常跟that引导的同位语从句，来对名词进行补充说明。句意：研究人员已经发现清晰、有说服力的证据，开心是健康和长寿的关键。故选D。