Make it singular or plural?

Hi. Please help. Let's say there is a teacher who teaches his young students how to write the alphabet correctly, and on the sheets that he already have handed out to his students have multiple sets of three lines, of which the middle line is dotted. And he wants his students to pay attention to the dotted lines when writing the alphabet. How should he word these instructions? Should it be the singular "the dotted line" or the plural "the dotted lines"? Thank you for your help in advance.

Please pay attention to the dotted line (dotted lines?) on your sheet when writing the alphabet.