We will find that you are eligible for disability insurance benefits even though you are still engaging in substantial gainful activity, if—

(a) You are blind;

(b) You are age 55 or older; and

(c) You are unable to use the skills or abilities like the ones you used in any substantial gainful activity which you did regularly and for a substantial period of time. (However, you will not be paid any cash benefits for any month in which you are doing substantial gainful activity.)