LAPD中FLAG的作用是（）
A.说明消息的来源
B.标识一个消息的开始和结束
C.说明链路是否可用
D.没什么作用
参考答案：B
There is an obvious _______ between children spending too much time playing computer games and their bad grades at school.
A．possibility
B．connection
C．chance
D．cause
人们对“礼仪”的理解和认识都有哪些层面的涵义？
