Concept:Root hairs are the main structures through which young roots absorb water from the soil.
Explanation:In young root regions, root hairs greatly increase the surface area for water uptake.
As the root matures, these root hairs are lost from the older parts.
Also, the outer cells of older root regions become thickened and less permeable to water.
Older roots still contain xylem, and their phloem remains alive, so options A and C are incorrect.
Surface area alone is not the main limiting factor; the absence of root hairs is the critical reason.
Therefore, older parts of roots do not normally absorb water because they no longer have root hairs.
Answer:D. they lack root hairs