下列物质与空气混合,接触到明火,有发生爆炸危险的是( )①氢气 ②汽油蒸气 ③面粉尘 ④棉尘 ⑤天然气
计算( 3 +2)2013( 3 ﹣2)2014的结果是( )
He asked us ____ (give) him some money.
计算12a5b6c4÷(﹣3a2b3c)÷(2a3b3c3),其结果是( )