I am pretty sure that I know which word should fill this statement. Because they have similar chemical structures, morphine and other opiates are able to lock into receptor sites for endorphins. According to the definition and main functions of <span>endorphins, this option is the only suitable. Regards!</span>
Organisms must reproduce or their species will die off. Reproduction is what keeps our world vibrant and teeming with life. If we don't reproduce, the human race would die off, and that would be the end of the world.