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. Required: a. Self-employment income of $65,000. b. Self-employment income of $65,000 and wage income of $35,500. c. Self-employment income of $160,000. d. Self-employment income of $301,000