【题目】We all know that a smile is usually a sign ________ people feel friendly and happy.
A.when
B.how
C.that
D.if

【答案】C
【解析】句意：我们知道微笑通常是一个人感到友好和愉悦的标志。sign是先行词，people feel friendly and happy是解释说明sign的内容，而且从句中意思完整，不缺成分，故用that来引导。所以答案是C。