every day every week every month every year
I gave my students this multiple choice question: A scientist plans a project to determine the effect of rain on the formation of caves. An area of rock has been chosen as the site for the investigation. How often should the scientist measure the changes in the rock?

Only one answer is correct among these 4 choices:
- every day
- every week
- every month
- every year

Could you tell me which one is correct?

every year