What is a keystone species?

A. A species which makes up only a small proportion of the total biomass 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 organization

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 other species