What is the correct advantage of using both hard and soft pencils together?

Correct answer: They provide a wide value range from light construction lines to dark tones

The governing concept is pencil value range. Hard graphite grades make lighter, cleaner construction marks and resist becoming very dark, whereas soft grades deposit more graphite and create deep shadows. Using both allows an artist to move from pale guidelines through middle values to strong dark accents, often with pressure control as well. Thus option C is correct; the other choices wrongly claim identical tones, paper protection, or colour.