(2010·安徽师大附中高三二模) 21.Some of the exercises appear to be _____ones that you have done, but after taking ____second look, you will find that they are different. A. /, the B. the, the C. the, a D. /, a 答案:C下列关于氧气的说法正确的是 ( )A. 氧气能支持燃烧,可作燃料 B. 带火星的木条一定能在含有氧气的集气瓶中复燃C. 氧气能跟所有物质发生氧化反应 D. 水中的生物能依靠微溶于水中的氧气而生存