【求助】白勇上的一道时态题

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 major role in health-care inflation.
(A) amounts to a sum lower
(B) amounts to less
(C) amounted to less
(D) amounted to lower
(E) amounted to a lower sum







答案是c
但我觉得题干中的last year有歧义
它可以指整个前半句发生在last year，即用Amounted
也可以指$541 billion发生在last year吧
这样的话，不是也可以用amounts吗？


请大家指点指点