The function of nonverbal signs is sometimes indispensible.For example， placing your index finger to your lips as an alternative to saying “Please calm down so that I can speak” is a case of（）.

The function of nonverbal signs is sometimes indispensible.For example， placing your index finger to your lips as an alternative to saying “Please calm down so that I can speak” is a case of（）.
A.complementing
B.repeating
C.contradicting
D.substituting
正确答案：substituting