John and Carole file a joint return and have three children: Jack, age 23; David, age 20; and Kristen, age 15. All three children live at home the entire year. Below is information about each of the children: 
• Jack: graduated from college last year and will start medical school next year. This year, Jack worked sparingly as he studied for the medical school entrance exam, but did earn $5,000. John and Carole provided 80% of Jack’s support during the year.
• David: a full-time student at State U., earned $6,400 from a part-time job, and provided 40% of his own support.
• Kristen: a full-time student in high school, had no gross income, and provided none of her own support.
a. Based on the above facts, which of the children can be claimed by John and Carole this year?
b. How would your answer to Part a change if Jack began medical school this year?
c. How would your answer to Part a change if Jack earned $3,000 rather than $5,000?
d. How would your answer to Part a change if David was a part-time student rather than a full-time student?
e. How would your answer to Part a change if David provided 60% of his own support rather than 40%?