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.