Why does a join line form when work is stopped midway through a long wet strip?

Correct answer: The first edge dries and separates from the next stroke

The governing concept is the continuous wet-edge technique. When work stops midway, the exposed edge begins to dry and loses its ability to merge smoothly with the next stroke. The new paint then meets a partly dry boundary and creates a visible join line, so option A is correct. A clean brush, a large palette, or flat paper does not by itself create this mark. Keep the working edge moist and complete a long strip without unnecessary pauses.