画出阶梯试块的外形图及各部尺寸。

在锌、铁、银、镁四种金属中，金属活动性最强是（　　）

A．锌

B．铁

C．镁

D．银

Middle born children will tell you that they usually didn’t feel all that special while growing up. The first born had his spot-carrier of the family banner and responsible for everything. The last born had his comfy little role, but the middle born had no distinctive place to call his own.

Middle-borns just seem to be easily overlooked, and maybe that’s why there are so few pictures of them in the family photo album. There may be hundreds, seemingly thousands, of pictures of the firstborn. For some strange reason, however, which I have confirmed by polling middle-born children around the world, there are seldom many pictures of the middle child, and what photos there are have him included with the others -- squeezed again between the older sibling and the younger sibling.

Another thing that can be said of many middle-born children is that they typically place great importance on their peer group. The middle child is well known for going outside the home to make friends faster than anybody else in the family. When a child feels like a fifth wheel at home, friends become very important; as a result, many middle children (but not all, of course) tend to be the social lions of the family. While firstborns, typically, have fewer friends, middle children often have many.

Middle children have a propensity to leave home first and live farther away from the family than anyone else. I observed a dramatic illustration of this tendency while I was a guest on Oprah Winfrey’s show. The subject that day was sibling rivalry. Three charming young women, all sisters, were among the guests, and we quickly learned that the firstborn and the last born were residents of the Eastern state where they had grown up. They had settled down near their parents and other family members. But the middle child had moved to the West Coast.

I suppose she could have gotten another two thousand miles farther away by moving to Hawaii, but her point was still well made. Middle children are the ones who will most often physically distance themselves from the rest of the family. It’s not necessarily because they’re on the outs with everyone else. They simply !ike to do their own thing, make their own friends, and live their own lives.

All of this is not to say that middle children totally ignore their siblings or the rest of the family. one common characteristic of the middle child is that she is a good mediator or negotiator. She comes naturally into this role because she’s often right in the middle, between big brother and little sister, whatever the case may be. And because she can’t have Mom or Dad all to herself, she learns the fine art of compromise. Obviously, these skills are assets in adult life, and middle children often become the best adjusted adults in the family.

What is the main argument of this passage？（）

A．First-born children are the carriers of the family banner and responsible for everything.

B．Middle-born children tend to be more independent and sociable while growing up.

C．Last-born children are the favorites who get most attention in the family.

D．Children are not really treated in the same way in the family.

计算机对声音采样一般采用采样频率、量化位数和______三个参数来衡量。

如果出口日用消费品、轻工业产品以及机械产品时常用的计算单位是：（）。

A、重量单位

B、长度单位

C、面积单位

D、个数单位

简述KGCfA型可控硅整流装置直流放大电路工作原理。