OG 77 PAGE 649 why less？

77. Even though the direct costs of malpractice disputes amounts to a sum lower than 1 percent of the $541 billion the nation spent on health care last year, doctors say fear of lawsuits plays a major role in health care inflation

C) amounted to less

D) amounted to lower

答案是C， 但是为什么？ Less 和lower 有什么区别，OG 解释说LESS 是名词，而lower 是形容词，而这里需要的是名词