A store sells a variety of sets for building molecular models. The sets are described by the diagram below. Each circled letter represents a box of 6 model atoms of one element. For example, the circled letter "P" represents a box with 6 model phosphorous atoms. Set 1 has 12 carbon, 12 oxygen, and 18 hydrogen atoms. Set 2 has 12 carbon, 12 oxygen, and 24 hydrogen atoms. Set 3 has 6 carbon, 12 oxygen, 12 hydrogen, 6 nitrogen, and 6 phosphorous atoms. Set 4 has 6 carbon, 6 oxygen, 6 hydrogen, and 6 phosphorous atoms.

Which change to one of the kits would make it useful for modeling proteins?

A. Set 1, add a box for phosphorous (P) atoms.
B. Set 2, add a box for sulfur (S) atoms
C. Set 3, replace the box of phosphorous (P) atoms with a box of sulfur (S) atoms
D. Set 4, replace the box of oxygen (O) atoms with a box of nitrogen atoms (N)