What is the most important precaution when combining different graphite pencil grades?

Correct answer: Avoid grooves from hard pencils and shiny burnished surfaces

Graphite grades differ in hardness and darkness. A hard pencil pressed heavily can indent the paper, while excessive layering or burnishing can fill the tooth and create a shiny surface that will not accept later graphite evenly. Option C gives the correct precaution: use controlled pressure and build darks gradually, often with softer grades. Equal pressure is not appropriate for every grade, and the other actions are unsafe or irrelevant.