Kara built a model of how molecules behave when they are in solid form. She filled a clear plastic box with marbles to the very top and put the lid on the box. When she shakes the box, the marbles can barely move. Which of the following is a limitation of Kara's model? A. The marbles have limited motion, but molecules in solids move a lot. B. The marbles are made out of atoms, but molecules are not. C. The marbles must be held in a box, but solids hold together without a container. D. The marbles are all the same shape, but molecules all have unique shapes.