请教OG 136中A选项两个it的指代问题

136.
                        Although schistosomiasis is not often fatal, it is so debilitating that it has become an economic drain on many developing countries.

(A)              it is so debilitating that it has become an economic

(B)              it is of such debilitation, it has become an economical

(C)             so debilitating is it as to become an economic

(D)             such is its debilitation, it becomes an econom­ical

(E)              there is so much debilitation that it has become
                    an economical

答案是A

有很多帖子讨论了C选项为什么错。但是我对A选项的两个it指代有疑惑，请NN指教：语法笔记说，同一句话中相同的代词，必须指代相同的事物，否则错。那么A选项中第一个it是形式主语，第二个it是指代血吸虫病，那么两个it的指代不一样，应该A也是错的啊，请教！！！！