问题
选择题
The researchers found clear and convincing evidence _________ happiness is the key when it comes to better health and longer lives.
A．where
B．whether
C．how
D．that
答案
D
题目分析：句意：研究人员发现清楚的令人信服的证据当说到健康和长寿的时候，快乐是关键。这里是that连接同位语从句，说明evidence的内容，选D。