Read the following statements about biodiversity.  In these statements, how many are related to Tilman’s long-term ecosystem experiments on outdoor plots?
(a) Increased diversity contributed to higher productivity.
(b) Species diversity decreases as we move away from the equator towards the poles.
(c) There were five episodes of mass extinction of species since the origin and diversification of life on the Earth.
(d) Plots with more species showed less year-to-year variation in total biomass.