求解答一语法题o_O o_O o_O

Cecile8chen

哎。。。白勇语法里时态第一题就错了 还有点不解。。题目如下：
Even  though the  direct costs of malpractice disputes amouts 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.

答案： amounted  to  less。
即原句正确为：
Even though the direct costs of malpractice disputes amounted to less 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.

书上解释说，last year 决定状语从句必须使用一般过去时。
我看了答案后也比较能理解价格需同一时间段比较 所以amount to要用一般过去式。
但是，我一开始做这道题的时候把the nation spent on health care last year看做修饰 the $541 billion ，所以选了amounts to less。请问这种理解错在哪。。。。。。

wyw1018

修饰没错，amount to 的主语是costs是复数，不能用amounts。