It is the idea smiles can get rid of cold has been proved by the researchers many times.
A. which; which
B. that; that
C. that; which
D. which; that
网友回答
A. which; which
(答案→)B. that; that
C. that; which
D. which; that
解析：第一that引导的同位语从句; 第二个that为强调句型的引导词that。