When performing subtraction by addition in the 2's-complement system:

• A. the minuend and the subtrahend are both changed to the 2's-complement.
• B. the minuend is changed to 2's-complement and the subtrahend is left in its original form.
• C. the minuend is left in its original form and the subtrahend is changed to its 2's-complement.
• D. the minuend and subtrahend are both left in their original form.