题目

Imagine having an idea，drawing it on paper，bringing it to a store and seeing it turned into a physical object．This is now possible with the help of 3D printers．Such machines were once used just by universities and big companies，but now，stores with 3D printing services are appearing around the United States． Bryan Jaycox and his wife opened The Build Shop LLC in Los Angeles two years ago．The store is filled with tools like a laser cutter，an industrial sewing machine and 3D printers．The Jaycoxs offer 3D printing classes and services for anyone who is interested． 我们一生都在不断吸入空气和呼出气体．在呼出的气体中，含量（体积分数）最多的气体是（　　） 　  A．             二氧化碳            B． 氮气            C． 水蒸气   D． 氧气