Answer: It could be A or B
Explanation: I´m having a little inner issue with the fact tha there are 2 parts of the volcano that match your question.
Crater: It could be considered as the central vent of the volcano. Some of these might be even as bigger as the whole stucture of the volcano.
Boiler: This could be define as a depression inside the crater, it could be very very deep, almost at the size of the whole volcano.
