The black tarnish that forms on pure silver is the sulfide Ag₂S, formed by reaction with H₂S in the atmosphere:

4 Ag + 2 H₂S + O₂ 2 Ag₂S + 2 H₂O

The sulfide forms in preference to Ag₂O, even though the atmosphere is 20% O₂ with just a trace of H₂S. Explain this by selecting all that apply.

a. Formation of the sulfide is consistent with the HSAB principle.

b. Silver is a soft Lewis acid.

c. Oxygen is a hard Lewis base.

d. Silver is a hard Lewis acid.

e. Hydrogen sulfide is a soft Lewis base.

f. Hydrogen sulfide is a hard Lewis base.

g. Oxygen is a soft Lewis base.

h. Formation of the sulfide is not consistent with the HSAB principle.