中央枢轴起定心和上下联接作用，挖掘机工作时，它承受（）和径向载荷都大。

In the following essay, each blank has four choices. Choose the best answer and write down on the answer sheet.

With the implementation of (1) , the so - called network has become a reality. The provision of such facilities is the most important part of the network requirements. However, in many applications the communicating computers may be of different types. This means that they may use different programming languages and, more importantly, different forms of (2) interface between user(application) program, normally referred to, as application processes, and the underlying communication services may be (3) . For example, one computer may be a small single - user computer, while another may be a large (4) system. In the earlier days of computer communication, these issues meant that only closed communities of computers (that is, from the same manufacturer) could communicate with each other in a meaningful way. IBM’s Systems Network Architecture (SNA) and DEC’s Digital Network Architecture (DNA) are just two examples of communication software packages produced by manufacturers to allow their systems to be interconnected together. These proprietary packages, however, of which there am still many inexistence, do not address the problem of universal interconnect ability, or open systems interconnection. In an attempt to alleviate this problem, (5) , in the late 1970s, formulated a reference model to provide a common basis (or the coordination of standards developments and to allow existing and evolving standards activities to be placed into perspective with one another.

3（）

A．same

B．similar

C．different

D．dependent

PTSD发生的病理学机制目前研究较多的主要集中在（）

A.PTSD神经影像学的研究

B.脑电生理学的研究

C.神经内分泌研究

D.神经免疫研究

E.人格特征

（8分）建筑工地有一块6000N的重物放在水平地面上，与地面的接触面积是50dm²．现用如图所示的滑轮组匀速提升，在重物上升0.8m的过程中，该滑轮组的机械效率为80%．（不计滑轮组摩擦及绳重）

则：(1)重物对水平地面的压强为多大？

(2)拉力F为多大？

(3)动滑轮重为多少？

(4)若提升的重物增加1500N，则此时滑轮组效率为多大？（解答要求：要写出依据的主要公式或变形公式，要有数据代入和运算结果）

在选择零件间的配合时，有相对运动的，应选择间隙配合，反之就选过盈配合

在设计架空避雷线时，高度上有什么规定，为什么？