在血平板上培养时会产生界限分明、无色透明的溶血环的微生物是（） A.肺炎链球菌 B.

In the past my hometown was used to be a beautiful place. Thick trees and green grass could be seen in everywhere. In order to build house and grow more crops, people cut down more and more trees. With time went on, the whole forest was almost destroyed. Gradually the green hills have changed into wasteland. As a result, sandstorms struck us now and then, of which we suffer a lot. However, I do hope all the people should realize the terribly result of not caring about our environment. What's more, they should take good care of the forests and plant trees instead cutting them down to improve our living conditions. 

目前1型糖尿病的主要死亡原因是

A．感染

B．酮症酸中毒

C．高渗性非酮症糖尿病昏迷

D．心脑血管病变

E．糖尿病肾病

下列说法正确的是（     ）

A．行星绕太阳的椭圆轨道可近似地看作圆轨道，其向心力来源于太阳对行星的引力

B．太阳对行星引力大于行星对太阳引力，所以行星绕太阳运转而不是太阳绕行星运转

C．万有引力定律适用于天体，不适用于地面上的物体

D．太阳与行星间的引力、行星与卫星间的引力、地球表面附近物体所受重力，这些力的性质和规律都相同

锅炉给水进行除氧的作用是（）。

A、去除水垢

B、防止氧腐蚀

C、去除钙、镁离子

D、降低硬度

三边孔是如何构成的？通过哪些结构？