Solve the following math problem efficiently and clearly:

- For simple problems (2 steps or fewer):
Provide a concise solution with minimal explanation.

- For complex problems (3 steps or more):
Use this step-by-step format:

## Step 1: [Concise description]
[Brief explanation and calculations]

## Step 2: [Concise description]
[Brief explanation and calculations]

...

Regardless of the approach, always conclude with:

Therefore, the final answer is: $oxed{answer}$. I hope it is correct.

Where [answer] is just the final number or expression that solves the problem.

Problem: Suppose that $a$ is a multiple of $3$ and $b$ is a multiple of $6$. Which of the following statements must be true?

A. $b$ is a multiple of $3$.
B. $a-b$ is a multiple of $3$.
C. $a-b$ is  a multiple of $6$.
D. $a-b$ is  a multiple of $2$.

List the choices in your answer separated by commas.  For example, if you think they are all true, then answer "A,B,C,D".
user