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

Researchers have found clear and convincing evidence happiness is the key to better health and longer lives.
A. where
B. whether
C. how
D. that

A. whereB. whetherC. 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。

这个答案应该是对的