帮忙看看这道语法题

OG673页语法第136题，

Although schistosomiasis is not often fatal, it is so debilitating that it has become an economic drain on many developing countries.

这道题就选A，那C选项so debilitating is it as to become an economic 为什么不对呢，so as to 不是可以表达和so that 一样的意思吗，而且新东方老师说这个结构用于简单句，那这不也正好吗？

建议mm可以搜索一下论坛:

http://forum.chasedream.com/dispbbs.asp?boardID=23&ID=24247&page=1

同时请mm发帖是按照格式发帖，在title里加入题号，并把题目帖全，以便后人搜索. 谢谢！

http://forum.chasedream.com/dispbbs.asp?boardID=23&ID=36320&page=1