The authority given the States to set the level of benefits payable under their AFDC plans stems from ?? 401 of the Social Security Act, 42 U.S.C. ?? 601 (1964 ed., Supp. IV), which states the purpose of the federal AFDC appropriations as 'enabling each State to furnish financial assistance and rehabilitation and other services, as far as practicable under the conditions in such State, * * *' (Emp