报告医疗器械不良事件应遵循什么原则？

阅读理解。

     Long, long ago there was no zero. To write the number sixty-three people wrote 63. To write six hundred

and three, people wrote 63. The space between six and three was there to mean." not any"tens. Sometimes

people did not remember the space. It was hard to see and read.

     Later people used a dot to hold the space. Six hundred and three looked like this 6.3. But the dot was hard

to see. So people put a circle around it like this 6⊙3.Then people could see the dot. They remembered the space. at last, only the circle around the dot was used. It was like a zero. This is one story of how the zero came to

be used. 

     Now zero has many important uses. Zero tells how many. Can you tell some other ways of using zero?

1. Long, long ago, people didn't know how to ______.

A. write

B. write zero

C. write numbers

D. sixty-three

2. Long, long ago if they wrote two hundred-eight, people wrote ______.

A. 28

B. 28

C. 228

D. 208

3. Later ______ was used to mean space.

A."not any"

B. letter "0"

C. zero

D. a dot

4. People used circles ______.

A. to remember ways

B. to remember numbers

C. not to forget the space

D. to mean nothing

5. The story tells us ______.

A. how zero came to be used

B. how to write zero

C. what`s the use of zero

D. that zero means a dot, a circle or space

x²=81，则x的值是（　　）

A．9

B．-9

C．9或-9

D．以上都不对

关于界面反射的描述，正确的是（）。

A.第二种介质的声阻抗大小决定反射声能的大小

B.反射角大于入射角

C.折射角小于入射角

D.折射的产生是入射角过小的缘故

E.声强反射系数为零时，超声将全部透射过界面

下列选项中，不属于眼球折光装置的是 ( )

A．虹膜

B．角膜

C．房水

D．晶状体

E．玻璃体

默认情况下，ICMP列表中（）。

A、不允许任何ICMP消息

B、允许所有ICMP消息

C、只允许传入的echo请求

D、不允许传入的echo请求