砾石充填防砂中常用()目的砂子。
参考答案:20~40
Tom speaks English very well, but he can speak ________ Japanese.[ ]
A. much
B. a lot of
C. little
D. a few
富氮原料发酵周期长;产气速度慢,富碳原料发酵周期短;产气速度快.