What is a keystone species?

• A
  $A$ rare species that has minimal impact on the biomass and on other species in the community.
• B
  $A$ dominant species that constitutes a large proportion of the biomass and which affects many other species.
• C
  $A$ 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.
• D
  $A$ common species that has plenty of biomass,yet has a fairly low impact on the community's organization.