For the majority of the compounds listed in Appendix C, the value of Gf is more positive

For the majority of the compounds listed in Appendix C, the value of ΔG°f is more positive (or less negative) than the value of ΔG°f.
(a) Explain this observation, using NH3(g), CCl4(l), and KNO3(s) as examples.
(b) An exception to this observation is CO(g). Explain the trend in the ΔG°f and ΔG°f values for this molecule.