问题补充：
A big factory has been built in this area
答案：
不定式 to 表目的
工厂被建在这块区域的目的是为了给整个国家人民制造这种物品.
我给你找的不定式to的用法
第2 第（6）的第①就是我所说的不定式 to 表目的
其他我也建议你看一下,好以后更容易做这类问题.
1．不定式的形式：（以动词write为例）
否定式：not + (to) do
（1）一般式：不定式的一般式所表示的动作与谓语动词动作同时发生或发生在谓语动词动作之后,
例如：Im glad to meet you.
He seems to know a lot.
We plan to pay a visit.
He wants to be an artist.
The patient asked to be operated on at once.
The teacher ordered the work to be done.
（2）进行式：不定式的进行式所表示的动作与谓语动词动作同时发生,例如：
The boy pretended to be working hard.
He seems to be reading in his room.
（3）完成式：不定式的完成式表示的动作发生在谓语动词动作之前,例如：
I regretted to have told a lie.
I happened to have seen the film.
He is pleased to have met his friend.
2．不定式的句法功能：
（1）作主语：
To finish the work in ten minutes is very hard.
To lose your heart means failure.
动词不定式短语作主语时,常用it作形式主语,真正的主语不定式置于句后,例如上面两句可用如下形式：
It is very hard to finish the work in ten minutes.
It means failure to lose your heart.
常用句式有：1、It+be+名词+to do.2、It takes sb.+some time+to do.3、It+be+形容词+of sb +to do.4、It+be+形容词+for sb.+to do.常用careless,clever,good,foolish,honest,kind,lazy,nice,right,silly,stupid,wise,等表示赞扬或批评的形容词,不定式前的sb.可作其逻辑主语.
（2）作表语：Her job is to clean the hall.He appears to have caught a cold.（3）作宾语：常与不定式做宾语连用的动词有：want,hope,wish,offer,fail,plan,learn,pretend,refuse,manage,help,agree,promise,prefer,如果不定式（宾语）后面有宾语补足语,则用it作形式宾语,真正的宾语（不定式）后置,放在宾语补足语后面,例如：Marx found it important to study the situation in Russia.动词不定式也可充当介词宾语,如：I have no choice but to stay here.He did nothing last Sunday but repair his bike.动词不定式前有时可与疑问词连用,如：He gave us some advice on how to learn English.（4）作宾语补足语：在复合宾语中,动词不定式可充当宾语补足语,如下动词常跟这种复合宾语：want,wish,ask,tell,order,beg,permit,help,advise,persuade,allow,prepare,cause,force,call on,wait for,invite.此外,介词有时也与这种复合宾语连用,如：With a lot of work to do,he didn go to the cinema． 有些动词如make,let,see,watch,hear,feel,have等与不带有to的不定式连用,但改为被动语态时,不定式要加to,如：I saw him cross the road.He was seen to cross the road.（5）作定语：动词不定式作定语,放在所修饰的名词或代词后.与所修饰名词有如下关系：①动宾关系：I have a meeting to attend.注意：不定式为不及物动词时,所修饰的名词如果是地点、工具等,应有必要的介词,如：He found a good house to live in.The child has nothing to worry about.
