What is biological nitrogen fixation

 It is a route in which N2 in the atmosphere is converted into ammonia by prokaryotes using an enzyme complex (Nitrogenase), and an N pool is maintained in the ecosystems, which replenishes N losses. BNF occurs naturally in the soil by N fixing bacteria (Rhizobium and legumes/Azotobacter)

stress fractures occur largely in the feet and legs and are common in sports that require repetitive impact, primarily running/jumping sports such as gymnastics or track and field.

i think that d. wind is the answer.   i missed the question when i answered a. currents.