题目

 Don't go out to play. You ________ finish your homework first. A. must B. have C. had D. had to 答案：A. 表示主观的必须用must。（2006?张家界）将带有火星的木条伸入盛有氧气的集气瓶中，看到的现象是木条复燃木条复燃，这一现象说明氧气具有助燃（或支持燃烧）助燃（或支持燃烧）的性质．