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
网友回答
A. where
B. whether
C. how
(答案→)D. that
解析：考察同位语从句.同位语从句that happiness is the key to better health and longer lives解释名词evidence的内容。