Instructions: In this task, you are given a context and four options. Each option is a suggested ending for the context. You should read the context and pick the best ending for the context. Please answer with "A", "B", "C", and "D". 
Input: [header] How to quickly multiply single digits by 9 [title] Remember that any number multiplied by 1 is the number itself. [step] No matter how tricky 9 is, it can't escape the fact that 1x9 will always equal 9. [title] Start simple and work your way up. <sep> (A) [step] If you're almost certain you know what you're doing, just jot down whatever you can think of. [substeps] Unless your calculations yourself require that you write them down first. (B) [step] Work your way up to 17 and you won't have a single digit. This will cause less confusion later! [title] Remember that most cell phones only have 14 digits. (C) [step] Don't try to multiply 9x9 before you learn 9x2 because it is a lot easier to think of math like a ladder. You should take each step to the top because if you try to skip steps you may just fall back and have to start over. (D) [step] You are at half the difficulty, meaning not even you can make the math as simple as you want it to be. In addition, there's a good chance that, once you understand fractions, you'll be able to figure out how to solve higher 9's with less work.
Output:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
C