You are building two forms. Each form has a button and a non-base table field. When the button is pressed, a complicated calculation is executed. The calculation is the same for both forms. The calculation code does not query or change the data in the database. When the calculation completes, it returns a single value to the non-base table field. To optimize performance and maintainability, which PL/SQL structure should be used? （）

• A、A trigger in each form. 
• B、A subprogram in each form. 
• C、A stored subprogram in the database. 
• D、A subprogram in a library that is attached to each form.