The researchers found clear and convincing evidence happiness is the key when it es to better health and longer lives.
A. where
B. whether
C. how
D. that
网友回答
A. where
B. whether
C. how
(答案→)D. that
解析：考查同位语从句。that解释前面的名词evidence，证据就是快乐是关键当谈到身体更健康寿命更长的时候。所以选D。