B. Roll-up cannot be performed on formula fields that use cross-object references or on-the-fly calculations such as NOW().

• This is because roll-up summary fields can only summarize data directly from child records to the parent record and cannot evaluate dynamic or cross-object data.

C. Roll-up summary can be performed on formula fields, but if their formula contains an #Error result, it may affect the summary value.

• While roll-up summary fields can summarize data from formula fields, any errors in the formula (such as division by zero) will affect the roll-up summary calculation.