What statement pertaining to the OAS program is true?

$0$ a$)$ Excluding exempt workers, contributions to the OAS program are mandatory for all employed individuals upon attaining age $18.$

$$b$)$ OAS benefits are adjusted quarterly to account for inflation.

$0$ c$)$ To apply for OAS benefits under the old rules, the applicant must have attained age $18$ as of July $1,1977.$

$$d$)$ A senior who applies for OAS benefits at age $68$ will receive benefits retroactive to age $65.$