地理兴趣小组对某小流域进行水土流失的调查。据此回答9～11题。

图4是几位同学根据调查结果绘制的坡度与单位面积产沙量关系曲线，其中符合水土流失一般规律的是（）

A．①

B．②

C．③

D．④

-Which tie is more suitable for me, the red one or blue one?

-I have no idea. You'd better take them __________.[ ]

A. all

B. each

C. every

D. both