Virginia is a cash-basis, calendar-year taxpayer. Her salary is $20,000, and she is single. She plans to purchase a residence in 2016. She anticipates her property taxes and interest will total $7,200. Each year, Virginia contributes approximately $1,000 to charity. Her other itemized deductions total approximately $800. For purposes of this problem, assume that 2016 tax rates, exemptions, and standard deductions are the same as for 2015. 
a. What will her gross tax be in 2015 and 2016 if she contributes $1,000 to charity in each year?
b. What will her gross tax be in 2015 and 2016 if she contributes $2,000 to charity in 2015 but makes no contribution in 2016?
c. What will her gross tax be in 2015 and 2016 if she makes no contribution in 2015 but contributes $2,000 in 2016?
d. Alternative c results in a lower tax than either a or b. Why?