What may happen if a second layer is applied over a wet first layer?

Correct answer: The two colours may mix unintentionally

The governing concept is wet-on-wet interaction. When a second colour is placed on a still-wet first layer, water carries pigment across the boundary, so the two colours may spread together and create an unintended blend. Option D is correct. The layer is not always cleaner, the colour does not instantly disappear, and the paper does not become thicker merely because another wet layer is added. Drying first is preferable when sharp separation is required.