A bird flies 'n' km one way. Which of the following expressions shows 2 round trips? (Remember that a round trip is a trip to a place and back to the starting place.)

• A.              \[\text{(4}-\text{n) km}\]                             
• B.              \[(\text{2}-\text{n) km}\]          
• C.              \[\text{(2n) km}\]          
• D.                                        \[\text{(4n) km}\]