Answer:
C. Winds carry air pollution across oceans and continents.
Planet Earth has complex global wind and air current systems. Thus, air pollution produced in one place, will never remain there for ever. It some cases, due to wind patterns, the majority of the pollution may be carried away to some other place on the globe. This is why US should be concerned about air pollution in China. Even if the distance between the two countries is signinificant, it does not mean that US will remain unaffected by China's activity.
