You can conditionally suppress sections based upon the result of another formula or a database field.
Under the Report menu, choose Section Expert.
Highlight Details B from the list on the left.
Click the X+2 button to the right of the "Suppress(No Drill Down)" property.
In the editor, setup your logic so that the logic evaluates to True or False.
Here is an example of the logic:
{table.field} = "A"
This will suppress the section when the field returns "A".
Save and Close.
repeat the process for Details C.
~Brian