Are chemicals released by the central nervous system that may provide natural relaxing and mood-elevating effects.

The chemical that is being described in the statement above is endorphins because this chemical is not only responsible of reducing pain but they cause the mood to be elevated in which these are being released by the central nervous system.

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.