<span>dense; expanding is the one</span>
Models help in the visualization and understanding of a phenomenon. For example, the atomic model helped scientists understand the arrangement of protons, neutrons and electrons in the atom long before it could be observed and this helped them relate the structure to its effects.
4 N to right
( the third option )
...................................
a). F= m a
500 N = (m) (5.00 m/s^2)
m = 100 kg
b). Accelerating body covers distance of
D = 1/2 a T^2
D = 1/2 (5.00 m/s^2) (100 s)^2
D = 25,000 meters
Note: I'm pretty sure the acceleration is typed wrong in the question. If so, then these answers won't be correct.