题目

Some students from Grade 9 ____ do some ____ for the old.  A. volunteered to; clean                                 B. volunteered; cleaning  C. volunteered to; cleaning                                    D. volunteered; clean 答案：C下列情况，a..煤的燃烧；b．工业废气的任意排放；c．燃放鞭炮；d．汽车尾气的排放；会引起空气污染的是A.aB.abC.abdD.abcd