解方程组3x-2y=12x+3y=-7．

短文改错：

下面短文中有10处语法错误。请在有错误的地方增加、删除或修改某个单词。

增加：在缺词处加一个漏字符号（∧）,并在其下面写上该加的词。

删除：把多余的词用斜线（\）划掉。

修改：在错的词下划一横线，并在该词下面写上修改后的词。

注意：①每处错误及其修改均仅限一词；

②只允许修改10处，多者（从11处起）不计分。

Did you enjoy yourself yesterday? I suppose you did. But I’d like to tell you how a bad day I had yesterday. Nothing just went wrong. In the morning, my alarm clock didn’t ring, so I woke up an hour late. I was in such hurry that I burnt my hand when I was cooking the breakfast. Then I ran out of the house to catch the 8:30 buses. This is the bus I usually take. But, you may laugh, I missed it of course. I am very unhappy and scared as well because teacher hates the students coming late to school. So I ran three miles to school only to discover that nobody was there. The school was quiet but peaceful. Our classroom was locking. I realized in that moment: It was Sunday.

遗传多态性是指基因组中任何位点上的相对差异以及所体现出来的遗传性状上的差异，主要包括()

A.多糖遗传标记

B.形态学遗传标记

C.蛋白质遗传标记

D.脂质遗传标记

E.DNA遗传标记

哪个是自顶向下确定企业实体的过程

A．实体分析

B．实体活动分析

C．实体间的联系

D．实体的确定

一对圆柱齿轮，安装后如果中心距过大会造成接触斑点偏向（）。

A、齿顶

B、齿根

C、中间

简述氧瓶燃烧法的原理、特点、应用范围、吸收液的作用及选择原则。