决定淡水生态系统初级生产量的限制因素（）
A.营养物质
B.光
C.水中氧气的浓度
D.食草动物的捕食
E.水浓度
参考答案：A, B, D
—Can I help you?
—_________. I want to buy some T-shirts for my children. [ ]
A. Yes, please
B. No, thanks
C. It doesn't matter
D. Of course I can
韶山4型电力机车轮对踏面偏差中修要求为（）。
A.≤0.5mm
B.≤0.6mm
C.≤0.7mm
D.≤0.8mm
