Why Do Plants That Live in Phosphorus-Poor Soils Produce Cluster

Why do plants that live in phosphorus-poor soils produce cluster roots?

A) because they are rare and produce carbonic acids, which solubilize the phosphorus complexes with iron and aluminum
B) because they are numerous and produce carboxylic acids, which solubilize the phosphorus complexes with iron and aluminum
C) because they are soft and produce carboxylic acids, which solubilize the phosphorus complexes with calcium and aluminum
D) because they are tough and produce carboxylic acids, which solubilize the phosphorus complexes with iron and calcium