Why Should a Power Analysis Be Done While Designing the Study

Why should a power analysis be done while designing the study (i.e., before data collection) ?

A) A power analysis will allow you to select the most appropriate alpha level for the study depending on your sample size.
B) A power analysis will tell you what effect size you should aim for in the data.
C) A power analysis will prevent misuse of the p value by putting the focus on confidence intervals instead.
D) A power analysis will let you know what sample size you need to have a good chance of detecting an effect if it exists.