开采层瓦斯抽放方式有哪几种？
参考答案：
开采层的瓦斯抽放有预先抽放、边采边抽（边掘边抽）和强化抽放三种方式。
— Dad,could you buy me a MP4 like this?
— Sure, we can buy ____ one than this, but ____this. [ ]
A. a worse, as good as
B. a better, better than
C. a cheaper, as good as
D. a more important, not as good as
同时喷射一般是曲轴每转一圈喷射（）油量。
A、1/2
B、2/3
C、2
D、1/4
