The reason h2s exists as a gas while h2o exists as a liquid at room temperature is because:
h2s has more electrons than h2o, causing a larger dispersion.
h2o has a lower boiling point than h2s.
h2s possesses hydrogen bonds, while h2o only possesses london dispersion forces.
h2o possesses hydrogen bonding, while h2s does not.

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.

the answer would be b i believe