What is the difference between S-ID.6a and S-ID.6c? If I have a lesson that addresses a standard but not all of the subsets of that standard, can I use just the standard? For example, if I’m working with S-ID.B.6 but do not include residuals in the lesson, can I use the standard (S-ID.B.6) instead of using the code S-ID.6a or S-ID.6c?