Acaldera forms when a- a volcano erupts b- a hot spot develops c- magma hardens in vertical cracks d- a volcano collapses

D- A volcano collapses

This is true, and is more specifically termed as periodic duty. This concept differs from other types of duty in that the load conditions are the same during each period, and the periods take place at intervals that are predictable. Other types of duty have intervals that are less certain.