Assign a possible reason for the following :
(a) Stability of + 5 oxidation state decreased and that of + 3 oxidation state increases down the group 15 elements.
(b) H2O is less acidic than H2S.
(c) SF6 is inert while SF4 is highly reactive towards hydrolysis.
(d) H3PO2 and H3PO3 act as reducing agents while H3PO4 does not.
(e) Helium gas is used by scuba divers.