Why is it necessary to clean the brush before blending?

Correct answer: To prevent unwanted colour mixing

A brush that still contains the previous colour can contaminate the next colour and spoil the intended blend. The result may become muddy, dull or different from the planned tone, so option B is correct. Cleaning the brush removes leftover pigment and gives better control over the transition. It does not make the brush heavy, wet the paper or dry the paint; those are separate actions or effects.