What is a keystone species?

Options

A. A species which adds up to only a smal proportion of the total biomas of a community , yet has a huge impact on the community's organization and survival
B. A common species that has plenty of biomass, yet has a fairly low impact on the community's organisation
C. A rare species that has minimal impact on the biomass and on other species in the community
D. A dominant species that constitutes a large proportion of the biomass and which affects many others species