What is a keystone species?

Options

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 organisation and survivial.
B. A common species that has plenty of biomass, yet has a fairly low impact on the community organisation.
C. A rate species that has a minimal impact on te biomass and on other species in the community
D. A dominant species that constituents a large proportion of the bimass and which affect many other species.