it was+形容词后面为什么能接for引导的从句?新概念第二册93课的第7题（选择题）原题如下：A site had to be found for it.it was necessary____for it(a)a site to be found (b)for to be found a site (c)for a site to be found (d)to be found a site我自已选的答案是a,后来核对发现c才是正确答案.

it was+形容词后面为什么能接for引导的从句?
新概念第二册93课的第7题（选择题）
原题如下：
A site had to be found for it.it was necessary____for it
(a)a site to be found (b)for to be found a site
(c)for a site to be found (d)to be found a site
我自已选的答案是a,后来核对发现c才是正确答案.