IV. Choose the word (A, B, C or D) that best fits the blanks space in the following passage.

Every day, the average person inhales about 20,000 liters of air. Every time we breathe, we risk inhaling ___(1)___ chemicals that have found their way into the air. Air pollution includes all contaminants ___(2)___ in the atmosphere. These dangerous substances can be either in the form of gases ___(3)___ particles. Air pollution can be found both outdoors and indoors. Pollutants can be trapped inside the buildings, ___(4)___ indoor pollution that lasts for a long time. These sources of air pollution are both natural and human- based. As one might expect, humans have been producing increasing ___(5)___ of pollution as time has progressed, and they now ___(6)___ for the majority of pollutants released into the air.