题目

 —I meant ______ you to the host at the party, but I forgot.    —That’s all right. Some other time, maybe.   A. to introducing      B. to introduce      C. introducing      D. having introduced 答案：B空气的成分按体积分数计算，大约是________占21%，________占78%，稀有气体占0.94%，二氧化碳占0.03%，其它气体或杂质占0.03%．