(a) S is less electronegative than O, therefore, it can donate a pair of electrons more easily than O. Thus, `HS^(-)` is a better nucleophile than `HO^(-)`.
(b) Since O-H bond is stronger than S-H bond, therefore `HO^(-)` has a greater tendency to accept a proton than `HS^(-)` and hence `HO^(-)` is a better base than `HS^(-)`.
