排污系数的物理意义是什么？
参考答案：
每耗用1t煤产生和排放污染物的量。
— Catherine, I've cleaned the room for you.
— Thank you. You _____ it. I could manage it myself. [ ]
A. needn't do
B. needn't have done
C. mustn't do
D. shouldn't have done
胡适的新文学运动中的第一部白话诗集是？
