In this task, you are given a context and four options. Each option is a suggested ending for the context. You should read the context and pick the best ending for the context. Please answer with "A", "B", "C", and "D".

[header] How to clean plaster walls [title] Use the dust brush attachment on your vacuum. [step] Attach the dust brush to the end of your vacuum and run the vacuum over the wall. This is the least abrasive way to clean walls and should always be your first step. <sep> (A) [substeps] It's okay if you use a suction tool if your vacuum's regularly coated with dust. Rock the vacuum back and forth continuously across the section of plaster to dislodge dirt, moss and grit. (B) Move the brush vertically across the wall, moving side to side. Try not to move the dust roller as straight up as you are going. (C) If you find that your walls are sufficiently clean after using the vacuum, you should stop after performing this step. [substeps] Do not put pressure on the wall as you vacuum it. (D) [substeps] Clean the walls after each use by applying grit or dirt. Residue collects in the air and can cause mold or mildew to grow on plaster walls.
C