问题
选择题
Scientists are convinced ________ the positive effect of laughter ________ physical and mental health.
A．of; at
B．on; at
C．by; in
D．of; on
答案
答案：D
题目分析：第一空考查短语be convinced of的用法，意为：相信。第二空on意为：在……方面。句意：科学家们相信笑对身体和精神健康有积极作用。