Unit 6. Part 4. Question 4. Watch the Unit 6 Part 4 lecture video and answer the following question: What is a caldera? Question 4 options: a. A large circular depression formed when the surface collapses into an emptying magma chamber. b. A crater formed on the top of a volcano by an explosive eruption the blows material outwards. c. A deep valley cut in the side of a volcano by a pyroclastic flow. d. A lake of lava the pools within the crater of a volcano