On Mars,there is no pollution ___ by traffic.A is caused B caused C that caused D that are caused

问题描述:

On Mars,there is no pollution ___ by traffic.A is caused B caused C that caused D that are caused

B,过去分词做定语

B caused ,因为这里是定语从句的省略,完整的句子应该是:
there is no pullution which/that is caused by traffic;