Many species of plants have nodes that are filled with nitrogen-fixing bacteria on their roots. The nitrogen that the bacteria "fix" is then used by the plant to build plant tissues. Where does most of the nitrogen that the bacteria use come from?

the correct answer is: b. pantala flavescens, because these organisms would have a high level of genetic variation due to sexual reproduction.  

because involves two organisms (parents) whose genetic material combines, sexual reproduction leads to genetic variation in new generations of offspring. the offspring is an unique organism, with new genome, more protected with the higher ability to adapted. this process is fundamental to evolution.

only about ten percent of the energy at one trophic level is passed on to organisms at the next trophic level. the remainder of the energy is used in life processes or lost as heat.