Concept:Xylem is a plant tissue that forms continuous hollow tubes for transporting water and minerals.
Explanation:Xylem cells are elongated and arranged end to end, forming long, narrow tubes.
At maturity, these cells lose their living contents and transverse end walls, leaving an open passage.
This uninterrupted tube reduces resistance to water flow, enhancing conduction of water and dissolved mineral salts.
Thickening with lignin mainly provides mechanical support and prevents collapse of the vessels.
Being close to the endodermis or centrally placed in the roots relates to tissue arrangement, not to conduction efficiency.
Therefore, the structural feature that directly enhances conduction is the tube-like alignment of xylem cells.
Answer:B. long, narrow tubes placed end to end