Is it okay to show just one way of performing step C?
Yes.
The relevant law is provided by 35 USC § 112(a), which provides (emphasis added):
The specification shall contain a written description of the invention, and of the manner and process of making and using it, in such full, clear, concise, and exact terms as to enable any person skilled in the art to which it pertains, or with which it is most nearly connected, to make and use the same, and shall set forth the best mode contemplated by the inventor or joint inventor of carrying out the invention.
As long as your description contains at least one example of how the invention could be put into practice, you will be fulfilling the requirement above. It is fine to note that alternatives may exist.
Can I indicate that other better methods exist for step C?
No.
The relevant law is provided by a different portion of 35 USC § 112(a), which provides: (emphasis added)
The specification shall contain a written description of the invention, and of the manner and process of making and using it, in such full, clear, concise, and exact terms as to enable any person skilled in the art to which it pertains, or with which it is most nearly connected, to make and use the same, and shall set forth the best mode contemplated by the inventor or joint inventor of carrying out the invention.
If you fail to disclose the best method that you know of, you would not be fulfilling your obligation under this section. While "best" can have a lot of different meanings depending on the context, it would be imprudent to explicitly note that you are not providing the best approach.
Conclusion
It is fine to disclose just one way of performing a step, as long as that is the best way.