I don't know how much the statement in the spec would help. You are basically saying, it would have been obvious to add C because C is conventionally part of a widget. That's something I guess, but not a lot. To me, assuming this is enough for an admission, the spec is just saying that you don't have to find a reference for ABC, that is known in the prior art. It's not really providing any reason to use C.
If I were formulating a rejection, assuming there is no explicit motivation, I would use one of the rationales set forth in MPEP 2143 I. It would seem like A, and maybe C and D, would apply, depending on what this actually is. Then I'd explain what the MPEP asks to explain.
(A) Combining prior art elements according to known methods to yield predictable results;
(C) Use of known technique to improve similar devices (methods, or products) in the same way;
(D) Applying a known technique to a known device (method, or product) ready for improvement to yield predictable results;
I understand this doesn't necessarily mean anything in litigation, but that is what I know.