Why is the nitrogen cycle considered a closed system?
a.
because nitrogen is fixed by bacteria and assimilated by plants
b.
because nitrogen neither enters nor leaves the system
c.
because nitrogen is converted into nitrates and ammonia
d.
because nitrogen is created by bacteria and destroyed by plants

Ten students stand in a circle and are told to make a transverse wave. what best describes the motion of the students? each student bumps the shoulder of his or her neighbor. at the exact time, all students take a step to the right. the students skip clockwise, going in a circular motion. one at a time, each student lifts his or her hands up and then down.

Imagine you had to physically add electrons, one at a time, to a previously neutral conductor. you add one electron very easily, but the second electron requires more work. in your initial post to the discussion, explain why this is. also, what happens to the work needed to add the third, fourth, fifth, and subsequent electrons

What are the north and south poles of a solenoid change with?