Answer:
An Atom's individual speed will change as it collides with other atoms, so we have to use an average.
Explanation:
In a gas a single atoms does an assortment of things during its time in the gas—sometimes it collides with an other atom gaining a lot of speed, sometimes losing a lot of speed in the collision, and sometimes just moving freely. Therefore: the motion of one individual atom is unpredictable, and it cannot be representative of all the the atoms in a gas, which is why we must average over all speeds of all atoms to find an average speed that allows us to calculate other quantities like temperature and pressure of the gas.
Hence, the second option "an Atom's individual speed will change as it collides with other atoms, so we have to use an average" stands correct.
