题目内容：

People always shake hands and say “How do you do?” when ______ to each other.
A.introduced B.introducing C.to be introduced D.to be introducing
为什么

优质解答

应该选A.
完整的句子是：
People always shake hands and say “How do you do?” when they are introduced to each other.
意思是：被介绍给别人的时候,人们经常会摆着手说：”.
这里省略了they are,直接填introduced就行.