What is the subtle difference between line and boundary?

Correct answer: A line can also give direction and feeling, while a boundary mainly shows separation

Direct answer: B. A line is a broad visual element that may show direction, movement, energy, texture, emotion, or an edge. A boundary is the separating edge between two areas, shapes, surfaces, or spaces. Sometimes a boundary is drawn with a line, but the two words describe different functions. To understand the difference, ask what the mark is doing. If it mainly separates an inside from an outside or one area from another, it functions as a boundary. If it leads the eye, suggests movement, creates rhythm, or expresses a feeling without simply separating areas, it functions as a line in a wider sense. A is wrong because a line can have many purposes beyond separation. B is correct because it compares the broader expressive role of line with the separating role of boundary. C is wrong because boundaries may be clearly visible. D is wrong because line is a mark or path, not merely colour. Memory cue: boundary answers ‘where does one area end?’; expressive line can also answer ‘where should the eye move?’