求救大全676！！！

大全676为何选A? A中那个it 是否会指代不清楚？因为前面有Sulfur dioxide 及pollutant. 谢谢！！！

676.Sulfur dioxide, a major contributor to acid rain, is an especially serious pollutant because it diminishes the respiratory system’s ability to deal with all other pollutants.

(A) an especially serious pollutant because it diminishes the respiratory system’s ability to deal

(B) an especially serious pollutant because of diminishing the respiratory system’s capability of dealing

(C) an especially serious pollutant because it diminishes the capability of the respiratory system in dealing

(D) a specially serious pollutant because it diminishes the capability of the respiratory system to deal

(E) a specially serious pollutant because of diminishing the respiratory system’s ability to deal

代词不是就近指代，而是

1）主从句中：先指代主语核心词，再宾语或表语的核心词，最后修饰语中的名词

2）并列句中：有2或3个句自并列，第2，3个句子的代词应优先指代第1句中主句的名词