---for their help ,we would not have overcome yhe diffculties.a were not b had it not been c had not it been d had it been not 为什么不选c请详解

因为had是助动词，而been是实意动词，英语在构成否定时是要否定助动词的，就是说not这个副词要紧跟在had的后面；而不像汉语那样直接用not去否定实意动词的。

此举考察的是虚拟语气的倒装，省略了if，还原应该是if it had been not for their help , we would not have overcome yhe diffculties.

虚拟语气的用法.虚拟语气可以分为三种,分别是
1,对过去的事情相反的虚拟
If 主语 had done sth,主语 should/would/might/could(not) have done sth.
2,对现在事情相反的虚拟
If 主语 did sth,主语 should/would/might/could(not) do sth.
3,对将来事情相反的虚拟（此种用法前面一个分句的用法较多,就不一一写出来了.）
这道题是对小类1虚拟语气的考察,在对过去事情进行虚拟时,可以省略if,与此同时,要把had提前,但是记住,只能提前had,其余的成分都不能提前.而且此种用法只能在第一类虚拟中才能使用,第二第三种是不能省略if的.

b