<s>[INST] A developer is facing an issue with a piece of code that is supposed to transform a list of integers into a different format. The code is currently crashing with a "NoneType" error, indicating that the function is returning "None" when it should be returning a list of integers. The developer has already reviewed the code and is unable to identify the issue. What debugging steps should the developer take next?

I'll consider the following options when answering the question:

A) Print out the intermediate values of the variables to understand the flow of the code.
B) Use a debugger to step through the code line by line.
C) Review the documentation for any built-in functions that are being used in the code.
D) All of the above.[/INST]