Teacher: You are given a paragraph, a question and some answer options (associated with "A", "B", "C", "D", etc.) You need to list all the correct answer options with their associated letters (e.g. "A" can be a valid answer). Note that sometimes, more than one option can answer the question correctly and completely. In those cases, please generate all such options separated by a comma (e.g. "A, B" is a valid answer). Do not generate anything else apart from one or more of the following characters: 'A', 'B, 'C', 'D', etc.
Teacher: Now, understand the problem? If you are still confused, see the following example:
Paragraph- Sent 1: Obama was born on August 4, 1961, at Kapiʻolani Maternity & Gynecological Hospital in Honolulu, Hawaii.
Sent 2: He is the only President to have been born in Hawaii.
Sent 3: He was born to a white mother and a black father.
Sent 4: His mother, Ann Dunham (1942-1995), was born in Wichita, Kansas, of mostly English descent, with some German, Irish, Scottish, Swiss, and Welsh ancestry. Question: How old was Obama's mother when he was born? (A) teenager (B) in his 40s (C) mid 20s (D) almost twenty.
Solution: D.
Reason: Obama was born in 1961. His mother was born in 1942 and 1961-1942=19. Therefore, "almost twenty" answers the question sufficiently.

Now, solve this instance: Paragraph- Sent 1: Heat supplies the energy that causes motion.
Sent 2: The deeper you go into toward the Earths core, the hotter it gets.
Sent 3: At the core it is really hot.
Sent 4: All that heat tries to rise toward the surface.
Sent 5: In the Mantle, the rock is partially melted and is able to move.
Sent 6: As it is heated, the material in the mantle moves toward the surface.
Sent 7: As the mantle material rises, it cools.
Sent 8: When it reaches the Earths crust, it is mostly stopped.
Sent 9: A little of the material can break through the surface, but not all.
Sent 10: Instead, it begins to move horizontally.
Sent 11: The mantle material moves horizontally away from a mid-ocean ridge crest.
Sent 12: Toward the surface, the mantle material starts to cool.
Sent 13: As it cools it sinks back down into the mantle.
Sent 14: These areas are where deep sea trench occur.
Sent 15: The material sinks back down to the core.
Sent 16: The system operates like a giant conveyor belt.
Sent 17: The motion due to heating and cooling is called convection. 
Question: What happens when the mantle material that is near the surface starts to cool? (A) It sinks back down into the mantle (B) It continues going toward the surface (C) It starts moving horizontally (D) It stops.
Student:
A.