单选题 Nowhere in nature is aluminum found free, owing to its always ______ with other elements, most commonly with oxygen.
A. combine B. combined
C. being combined D. having combined

【正确答案】 C
【答案解析】[解析] owing to是介词短语,后面应跟名词或动名词,its being combined with other elements为动名词短语。其他三个选项不合适,因为combine为原形动词,combined为过去分词,having combined为主动态。