第12题: [单项选择]Nowhere in nature is aluminum found free, owing to its always ______ with other elements, most commonly with oxygen. A. being combined B. having combined C. to combine D. combined 参考答案:A 答案解析:句意:由于铝总是和其他元素结合在一起,最常见的是和氧气在一起,所以自然界中哪里也不会找到单独的铝元素。owing to是介词,后应接名词或动名词,根据题意,应接被动语态形式。