消减断面主要反映河流对污染物的稀释净化情况，应设置在（）下游，主要污染物浓度有下（）降处。
参考答案：控制断面；显著
句型转换
1. Mr Hu taught her math last year.(改为一般疑问句)
_____ Mr Hu _____ her math last year?
2. His friend took the photos yesterday.(改为否定句)
His friend _____ _____ the photos yesterday.
3. Tom often goes skating on Sundays in winter.(改为一般疑问句)
_____ Tom often _____ skating on Sundays in winter?
4. She doesn't wash her clothes on Sundays.(用last Sunday代替on Sunday并改写句子)
She _____ _____ her clothes last Sunday.
5. Were there any actors at the theater? (作否定回答)
No, _____.
不宜采用样方法调查种群密度的是（）
A.荠菜
B.蚜虫
C.蒲公英
D.菜粉蝶
