氧气中的()的总含量达到或超过2﹪(体积分数),禁止装瓶。
A、乙炔
B、乙烯
C、氢
D、氮
参考答案:A, B, C
A good story does not necessarily have to have a happy ending, but the reader must not be left . [ ]
A. unsatisfied
B. unsatisfying
C. to be unsatisfying
D. being unsatisfied
二手车交易基本方式有:()、销售、寄售、代购、代销、租赁、拍卖、置换等。