又遇到难题了。。。请大神解释。。。

Nowadays the Chinese are becoming more familiar with different cultures and other ways of doing things，____is often the case in other countries.
A.as B.what C.which D.that
为什么只能选A不能选C，C不是也可以引导非限制性定语从句吗？

就如另外一位网友所说，"as is the case/ as is often the case" 的确是固定搭配, 用来表示某种情形是常见的。例如 －
1. Leon is very mischievous, as is often the case with children./ As is often the case with children, Leon is very mischievous.
2. Lgc398423666 is very eager to improve his English, as is often the case with students from China./ As is often the case with students from China, Lgc398423666 is very eager to improve his English.
所以说在这道题里，我们用这个搭配指出 “华人现在对别的文化和习俗越来越熟悉了” 的这个趋势在很多别的国家也同样在发生。


其实在这句话里用which 的话，句子也是通顺的，语法完全没有问题。不过我想老师在这题当中可能想考的是你知不知道 "as is often the case" 是一种常见的搭配。用A 的确比较顺，而做选择题时本来就该选最合适的。不过请放心，就算填 which, 句子也没有语法问题。(:


还有，我顺便提醒你，如果句子结构换了，就不能互换 which 和 as。－
1. 对的－As is often the case in other countries, the Chinese are becoming more familiar with different cultures and other ways of doing things nowadays. 
2. 错的－Which is often the case in other countries, the Chinese are becoming more familiar with different cultures and other ways of doing things nowadays. 


所以说还是不要拆散固定搭配，做选择题也没那么危险！哈哈，希望帮到你了。继续加油！

as is ofen the case是固定搭配，as指代前面整句话，短语意为“。。。很常见”，记住就是了，下次遇见就会选了

用vlookup,如果是2007版本以上最好外套一个iferorr 以下图为例