Given the task definition and input, reply with output. 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.

Paragraph- Sent 1: I'm here to tell you the story of a robot named Carl.
Sent 2: He came from a far away land known as Factory.
Sent 3: Carl was sad because he was missing a part called a tire.
Sent 4: He also needed a sun gatherer.
Sent 5: But, the tire was more important.
Sent 6: Once Carl got all these parts he could travel to his new home in the nation of Lab and the city of Office.
Sent 7: It was a tricky thing to get there with missing parts.
Sent 8: Just as he had given up hope Carl got a message from Mr. X saying the new parts were ready to be delivered.
Sent 9: This made the robot very happy.
Sent 10: The parts arrived a few days later and Carl put them in with 2 days of work.
Sent 11: After this Carl began to travel the last bit of his goal to get to his new job.
Sent 12: After this Carl took 10 days to get to Lab.
Question: Which was the most important missing part that Carl has to get after getting out of the factory? (A) A tire (B) Tire and sun gatherer (C) Sun gatherer (D) A round tire.
A, D.