单选题
8.Nowhere in nature is aluminum found free, owing to its always______with other elements, most commonly with oxygen.
【正确答案】
C
【答案解析】owing to是介语短语，后面应跟名词或动名词，its being combined with other ele-ments为动名词短语。其他三个选择答案不合适，因为combine为动词原形；combined为过去分词；having combined为主动态。