The free energy of formation (∆fG°) of CO from C becomes lowerat temperatures above 1120 K whereas that of CO2 from C becomes lowerabove 1323 K than (∆fG°) of ZnO. However, (∆fG°) of CO2 from CO is alwayshigher than that of ZnO. Therefore, C can reduce ZnO to Zn but not CO.Therefore, out of C and CO, C is a better reducing agent than CO forZnO.
Reduction of Zn is usually carried out around 1673 K,
ZnO+ C

1673K

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Zn+ CO