A.1:1 B.1:10 C.1:25 D.1:50
A.0° B.10~20° C.90° D.30~45°
A.水體無(wú)污染 B.水體正在自凈 C.水體已經(jīng)自凈 D.不能說(shuō)明問(wèn)題