Which organisms convert nitrogen into a form that is usable by humans

Giraffes with short necks tend to be unable to access enough food, while giraffes with long necks can reach more food at the tops of trees. this causes an advantage for giraffes with long necks. how would you explain this? a. this is an example of a stabilizing selection in which the intermediate form of a trait is advantageous compared to the extreme forms of the trait. b. this is an example of a disruptive selection in which the population of cacti is divided and well-defined. c. this is an example of directional selection in which one end of the bell curve is better adapted to its environment than the other end. d. this is an example of bidirectional selection in which evolutionary pressures from multiple sources encourage an organism to adapt