(a) General. In general, the substantial services test will be applicable only in a grace year (including a termination grace year) as defined in § 404.435(c)(1). It is a test of whether, in view of all the services rendered by the individual and the surrounding circumstances, the individual reasonably can be considered retired in the month in question. In determining whether an individual has or has not performed substantial services in any month, the following factors are considered:

(b) Individual engaged in more than one trade or business. When an individual, in any month, performs services in more than one trade or business, his services in all trades or businesses are considered together in determining whether he performed substantial services in self-employment in such month.

(c) Evidentiary requirements. An individual who alleges that he did not render substantial services in any month, or months, shall submit detailed information about the operation of the trades or businesses, including the individual's activities in connection therewith. When requested to do so by the Administration, the individual shall also submit such additional statements, information, and other evidence as the Administration may consider necessary for a proper determination of whether the individual rendered substantial services in self-employment. Failure of the individual to submit the requested statements, information, and other evidence is a sufficient basis for a determination that the individual rendered substantial services in self-employment during the period in question.