Ben helped his parents load boxes and furniture onto the moving truck. He easily carried several boxes out of the house and up a ramp into the truck. Then loaded small pieces of furniture by pushing them up the ramp. Finally, Ben tried to move the refrigerator. He pushed and pushed, but it did not move. Why?
A) because the refrigerator was so tall
B) because there was no wheels on the refrigerator
C) because Ben was tired from moving the boxes and furniture
D) because the refrigerator was much heavier than the other objects
