已知xa=3，xb=5，则则x3a-2b=（ ）
有下列物质：①氧气；②氮气；③五氧化二磷；④二氧化硫；⑤二氧化碳；⑥食盐水；⑦氖气．请用这些物质的数字代号填空：
你认为原子结构与下列事物结构最接近的是（ ）
— They are Mary's bag. She left it on the playground just now, but now she can't find it.
