定向邮品的价格取决于四个因素：邮品设计要求、印刷数量、邮票选择和()要求。
参考答案：工艺材料
—Thanks a lot for helping me.
—_____[ ]
A. You are welcome.
B. Yes, I see.
C. No, thank you.
D. Oh, that's very kind of you.
平流沉砂池流量最大时，废水在池内的停留时间不小于（）
A.10s
B.15s
C.20s
D.30s
