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？