Chloride resides mainly in fluid, and is critical for transmitting nerve impulses and for maintaining acid-base balance.
a. extracellular
b. intracellular
c. intercellular
d. extercellular

they are made of gases.

they have a spherical shape.

they make energy through fusion.

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.