天然气输送系统中的液体和固体杂质主要来自三方面Liquid and solid impurities in natural gas transportation system mainly come from three aspects：（）

A、采气时井下带来的凝析油、凝析水、岩屑粉尘Condensate oil, condensate water and cuttings dust brought by underground gas extraction
B、管道施工时留下的脏物和焊渣Dirt and welding slag left by pipeline construction
C、管内的锈屑和腐蚀产物Rust and corrosion products in the tube
D、其他