How do the horizontal portions of the graph represent? When heat is added or removed from the substance and the temperature stays the same, where is the thermal energy going, Why?

The comparison of the number of atoms in a copper coin the size of a penny with the number of people on earth is made to illustrate which of the following? a. that atoms are indivisible b. that atoms are very small c. that atoms are very large d. that in a copper penny, there is one atom for every person on earth