Models need to be changed when

a.) they are based on experimental data.
b.) scientific understanding changes.
c.) they represent a phenomenon.
d.) they are designed to answer a question

I think the correct answer from the choices listed above is option B. Models need to be changed when scientific understanding changes. This should be done since the assumptions where the models are based on are changed. Hope this answers the question.

Droughts cause very arid places such as a desert