Determine the maximum contribution that can be made to a Keogh plan in each of the following cases. In all instances, the individual is self-employed, and the self-employment tax reduction has already been taken.

a) Self-employment income of $46,000.

b) Self-employment income of $46,000 and wage income of $30,000.

c) Self-employment income of $125,000.

d) Self-employment income of $290,000.