Dale makes a model of part of an elemental solid by attaching balls to each other with rods. his model looks something like the following image:

how could dale improve his mode

a.

by using springs instead of rods to allow the model atoms to vibrate in their positions

b.

by using strings instead of rods to allow the model atoms to move more freely

c.

by using different sizes of balls to represent different kinds of atoms

d.

by using cotton balls instead of hard balls to allow the model atoms to change size