As more and more cars are produced and used,so the _____ fro

答案D
　　emission散发物,发射.exposure(to)暴露于：They believe that their health problems are the result of years of exposure to fine cotton dust.(他们认为,他们的健康问题是多年来暴露于棉花粉尘所致.)expansion膨胀,扩大；Heat causes the expansion of gas.本题译文：正如汽车的生产和使用越来越多一样,汽车的排气管的散发物内所含的有毒气体量也在增加.