he is __(迷惑)about his future.

句1：emitted from vehicles 是修饰gas的定语,gas是被emit的,所以用被动.
句2：resulting from the improvement of technology and science 是修饰quality的定语,这里用主动与result from短语的特点有关,A result from B 的意思是 A由B导致,（from嘛）,无被动,所以用主动.
总的来说,被修饰的中心词是主动怎么怎么样的就是用主动,被怎么怎么样的就是用被动.

多出来的is是谁给抄错了.原文是一套测验题,正确的句子是：
This it certainly is,but this is by no means its only achievement,for there are many suggestions for further lines of research,and the discursive passages are crisply provocative of new ideas and new ways of looking at established findings.
所以你第二个问题不存在,只有一个is.要回答第一个问题,需要看这句前面的一句.我给你加上：
As a result,the book seems at first sight to be an exhaustive and critically useful review of the literature.This it certainly is,but this is by no means its only achievement,for there are many suggestions for further lines of research,and the discursive passages are crisply provocative of new ideas and new ways of looking at established findings.
这样看,This it certainly is中的This是指前一句的表语,而it指的是the book.这是个倒装句,翻过来就是it certainly is this.把两个代词用它们所代指的替换,就是The book certainly is an exhaustive and critically useful review of the literature.也就是说,这句话把前一句基本上重复了一遍,只不过改变了强调并且更加肯定.倒装的原因是为了使this更清楚地指向前一句的表语.

一.B 此处和alert共用一个stay,而stay+adj/分词表示"保持一种...状态"
二.A grow Vt.种植,培育(=plant),有被动语态
grow还可以做Vi 生长,此时无被动语态
如果选D表种植,就应该用过去时,因为它已经拿到展销会来了,种植/培育动作早就完成了.
三.B short adj 它不能单独做主语啊,要用就该用Being short of
而shortage才是名词,可做句子主语
四.我来解释一下lack of和short of的区别
1.short adj be short of...缺少...
2.lack
1) n.(a) lack of...缺少...for lack of...因为缺少...
2)Vt.lack sth/sb 缺少...
3)Vi lack for sth 缺少...,但是只用于否定句
另外:lacking adj.be lacking in...缺少...
3.所以第4题只能选择A.
原因:lack(n.) of...用在例句
The school is quite lack(adj?/n!) of teachers肯定不通顺
可以这么造句,比如:Despit (a) lack of confidence,he still made an attempt.