(a) The State agency will secure from the claimant, or other sources, any evidence it needs to make a disability determination.
(b) We will secure from the claimant or other special arrangement sources, any evidence we can obtain as adequately and more readily than the State agency. We will furnish the evidence to the State agency for use in making a disability determination.
(c) At our request, the State agency will obtain and furnish medical or other evidence and provide assistance as may be necessary for us to carry out our responsibilities—
(1) For making disability determinations in those classes of cases described in the written guidelines for which the State agency does not make the determination; or
(2) Under international agreements with respect to social security benefits payable under section 233 of the Act.