Agenus is composed of a number of a) kingdoms b) phyla c)orders d)species

A genus is composed of a number of related species. A genus is a taxonomic rank in the classification of organisms between the family and the species.  A genus contains a group or related species of organisms exhibiting common attributes. A genus name constitutes the first part of the binomial species name for individual species within the genus. For example, Felis is a genus within the family Felidae. Felis catus and Felis silvestris are the two species within the genus.

any abiotic or biotic factor in an ecosystem that causes a population's size to slow or decrease is a limiting factor.