简述三相异步电动机几种电力制动原理？

In history, every great culture had its own ideas of beauty ______ in art and literature.

A．expressed

B．to express

C．being expressed

D．to be expressed

一个梯形的上底扩大3倍，下底也扩大3倍，高不变，梯形的面积就会______．

In the dark forests______, some large enough to hold several English towns.

A．lies many lakes

B．lie many lakes

C．do many lakes lie

D．does many lakes lie

（）是当轿厢或对重超过下极限位置时，用来吸收轿厢或对重装置所产生动能的安全装置。

A、安全钳

B、限速器

C、端站减速装置

D、缓冲器

下列表述中正确的是（）

A.平均数反映离散性

B.极差反映集中性

C.中位数反映离散性

D.众数反映集中性