简述同步电机的“同步”是什么意思？

下列关于机械波的说法，正确的是（　　）

A．波源振动越快，波传播的速度就越大

B．波的频率等于介质中各质点的振动频率

C．波传播的速度等于介质中各质点的振动速度

D．波中各个质点每经过一个周期沿波的传播方向移动一个波长

王平在小学时，学习成绩一直很优秀，同学们很佩服他，也经常受到老师的表扬。进入初中后，由于学习难度加大，竞争对手越来越多，王平的成绩下降了，老师对他的表扬越来越少了。王平从来没有过这样的感受，这无疑是一个重大打击，于是他便闷闷不乐，产生了辍学的念头……

（1）王平遇到挫折的主要原因是什么？

（2）请你告诉王平应该怎样对待挫折。

Directions:This section consists of three groups of pairs listed in two columns, A and B. You are to match the one marked 1), 2), 3) ,4) or 5) in Column A with the one marked a, b, c, d or e in Column B. One point is given to each pair you match.
1) ______ the Grammar-Translation Method
2) ______ the Direct Method
3) ______ the Audiolingual Method
4) ______ the Cognitive Approach
5) ______ the Communicative Approach
a) The primary function of language is communication.
b) All languages originated from one language and were ruled by a common grammar.
c) Language is a system of structurally related elements for the expressions of meaning.
d) Every language has its own structures and can not be forced into the constraining pattern of Latin grammar.
e) A native speaker has in his brain a set of grammar rules which he can use to make sentences with.

WHO确定的龋病诊断标准是

A．釉质上的白斑
B．釉质上的着色不平坦区
C．探针可插入着色窝沟，底部不发软
D．探诊窝沟底部发软，釉质有潜在损害或沟壁软化
E．釉质上硬的凹陷

全面结算会员期货公司向非结算会员收取的保证金，不得高于期货交易所向全面结算会员期货公司收取的保证金标准。( )