Income taxation problem

Kelsey is a cash-basis, calendar-year taxpayer. Her salary is $30,000, and she is single. She plans to purchase a residence in 2016. She anticipates her property taxes and interest will total $8,000 in 2016. Each year, Kelsey contributes approximately $1,500 to charity. Her other itemized deductions total $2,000. For purposes of this problem, assume 2016 tax rates, exemptions, and standard deductions are the same as 2015. What will her gross tax be in 2015 and 2016 if she contributes $1,500 to charity in each year? What will her gross tax be in 2015 and 2016 if she contributes $3,000 to charity in 2015 but makes no contribution in 2016? What will her gross tax be in 2015 and 2016 if she makes no contribution in 2015 but contributes $3,000 to charity in 2016? Why does alternative "c" yield the lowest tax