While reading, you’d better make a mark ___ you have any questions.

A．on which

B．at which

C．the place where

D．where

现行的《地表水环境质量标准》(GB 3838—2002)中，Ⅲ类水DO的标准值是( )mg/L。

A．≤5

B．≥5

C．≤3

D．≥6