Four similar boxes are labelled I, II, III, and IV. The masses of I and III are equal and the masses of II and IV are equal. The mass of box I is smaller than that of box II. Boxes I and II are placed in a grass field and boxes III and IV are placed on a marble floor. Which box requires the maximum force to be moved?

• A.  I                                             
• B.  II
• C.  III                                          
• D.  IV