OG 101-Less

101. Even though the direct costs of malpractice disputes amounts to a sum lower than one 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.

(A)  amounts to a sum lower

(B)  amounts to less

(C)  amounted to less

(D)  amounted to lower (D)

(E)   amounted to a lower sum

答案没有问题。只是问，OG解释中，

In D, the adjective lower is erroneously used in place of the noun less as object of the preposition to. 是什么意思？谢谢