一道超简单的题语法点探讨

下面这题已经翻来覆去做了好多遍了。我在重新整理的时候遇到一个问题，想和大家交流下。在选项A中的only位置和选项Conly的位置不同，A更合理因为强调的内容是只有两种疾病。但是从语法结构上来说only是个副词，应该是不可以修饰介词结构的。所以为什么这里的only又可以放在介词结构之前呢？这是属于什么用法呢？大家可以说说么？

Diabetes, together with its serious complications, ranks as the nation’s third leading cause of death, surpassed only by heart disease and cancer.

(A) ranks as the nation’s third leading cause of death, surpassed only

(C) has the rank of the nation’s third leading cause of death, only surpassed