What conclusion can you draw from the data? a. all rhizobium strains have a similar, highly positive effect on plant growth. b. some rhizobium strains are much more effective at promoting plant growth than other strains; the most ineffective strains have little positive effect. c. some rhizobium strains are much more effective at promoting plant growth than other strains; however, even the most ineffective strains have a significant positive effect. d. none of the rhizobium strains used in the study have a substantial positive effect in promoting plant growth.