In a flat rectangle, the value has become lighter in the centre and darker at the edges. What is the main cause?

Correct answer: Uneven distribution of paint and moisture

The governing concept is an even flat wash. If the brush carries different amounts of paint or water, the centre may receive a thinner film while liquid and pigment gather near the boundaries, producing a darker edge and lighter middle. Option A is correct. Correct paper, uniform strokes, and enough premixed colour generally support consistency rather than create this pattern. Keep the brush load steady, work before areas dry, and avoid pushing wet pools toward the edges.